Add configurable CTAN repository with validation and version detection

Allow manual workflow runs to specify CTAN mirror via workflow_dispatch input.
Validate repository accessibility and detect TeX Live version before build starts.
Include repository URL and version in release notes for tracking yearly updates.

This will help with situation like #50 were some mirrors may not be updated

Author: cderv

.github/workflows/build.yaml

@@ -1,5 +1,11 @@
 on:
   workflow_dispatch:
+    inputs:
+      ctan_repo:
+        description: 'CTAN repository URL (e.g., https://ctan.math.illinois.edu/systems/texlive/tlnet). See https://ctan.org/mirrors/mirmon for available mirrors.'
+        required: false
+        type: string
+        default: 'https://ctan.math.illinois.edu/systems/texlive/tlnet'
   schedule:
     - cron: '5 3 * * *'
 
@@ -11,9 +17,68 @@ env:
   # required for GH to run on another repo (-R option)
   # https://cli.github.com/manual/gh_help_environment
   GH_REPO: rstudio/tinytex-releases
+  # CTAN repository URL for TeX Live packages
+  CTAN_REPO: ${{ inputs.ctan_repo || 'https://ctan.math.illinois.edu/systems/texlive/tlnet' }}
 
 jobs:
+  validate-ctan:
+    runs-on: ubuntu-latest
+    name: Validate CTAN repository
+    outputs:
+      texlive-year: ${{ steps.check-repo.outputs.texlive-year }}
+    steps:
+      - name: Check CTAN repository accessibility
+        id: check-repo
+        run: |
+          echo "Validating CTAN repository: $CTAN_REPO"
+
+          # Check if the repository is accessible by fetching the texlive.tlpdb file
+          if ! curl --fail --silent --show-error --head "$CTAN_REPO/tlpkg/texlive.tlpdb" > /dev/null; then
+            echo "✗ CTAN repository is not accessible or invalid"
+            echo "Please check the URL: $CTAN_REPO"
+            exit 1
+          fi
+
+          echo "✓ CTAN repository is valid and accessible"
+
+          # Detect TeX Live version by checking for TEXLIVE_YYYY files
+          echo "Detecting TeX Live version..."
+          CURRENT_YEAR=$(date +%Y)
+          TEXLIVE_YEAR=""
+
+          # Check current year first, then previous year
+          for year in $CURRENT_YEAR $((CURRENT_YEAR - 1)); do
+            if curl --fail --silent --show-error --head "$CTAN_REPO/TEXLIVE_$year" > /dev/null 2>&1; then
+              TEXLIVE_YEAR=$year
+              break
+            fi
+          done
+
+          if [ -n "$TEXLIVE_YEAR" ]; then
+            echo "✓ TeX Live version: $TEXLIVE_YEAR"
+            echo "texlive-year=$TEXLIVE_YEAR" >> $GITHUB_OUTPUT
+          else
+            echo "⚠ Could not detect version with current/previous year, checking older versions..."
+            # Check for older TEXLIVE_YYYY files (going back 5 years)
+            for offset in {2..5}; do
+              year=$((CURRENT_YEAR - offset))
+              if curl --fail --silent --show-error --head "$CTAN_REPO/TEXLIVE_$year" > /dev/null 2>&1; then
+                TEXLIVE_YEAR=$year
+                break
+              fi
+            done
+
+            if [ -n "$TEXLIVE_YEAR" ]; then
+              echo "✓ TeX Live version: $TEXLIVE_YEAR"
+              echo "texlive-year=$TEXLIVE_YEAR" >> $GITHUB_OUTPUT
+            else
+              echo "⚠ Warning: Could not detect TeX Live version (no TEXLIVE_YYYY file found)"
+              echo "texlive-year=unknown" >> $GITHUB_OUTPUT
+            fi
+          fi
+
   new-release:
+    needs: [validate-ctan]
     runs-on: ubuntu-latest
     name: Create new daily draft release
     outputs:
@@ -35,7 +100,6 @@ jobs:
     name: Build Bundles For Windows
 
     env:
-      CTAN_REPO: https://ctan.math.illinois.edu/systems/texlive/tlnet
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
 
     steps:
@@ -111,7 +175,6 @@ jobs:
       tlmgr-version: ${{ steps.tlmgr-version.outputs.TLMGR_VERSION }}
 
     env:
-      CTAN_REPO: https://ctan.math.illinois.edu/systems/texlive/tlnet
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
 
     steps:
@@ -213,7 +276,6 @@ jobs:
     name: Build Bundles For macOS
 
     env:
-      CTAN_REPO: https://ctan.math.illinois.edu/systems/texlive/tlnet
       GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}
 
     steps:
@@ -322,7 +384,7 @@ jobs:
             gh release upload ${{needs.new-release.outputs.draft-tag}} tinitex.zip tinitex.tgz tinitex.tar.gz --clobber
 
   deploy:
-    needs: [new-release, build-windows, build-linux, build-mac, tinitex]
+    needs: [validate-ctan, new-release, build-windows, build-linux, build-mac, tinitex]
     runs-on: ubuntu-latest
     name: Publish new daily release
 
@@ -339,6 +401,12 @@ jobs:
           ${{ needs.build-linux.outputs.tlmgr-version }}
           EOF
           echo '```' >> notes.md
+          echo "" >> notes.md
+          if [ "${{ needs.validate-ctan.outputs.texlive-year }}" != "unknown" ]; then
+            echo "Built from CTAN repository: \`$CTAN_REPO\` (TeX Live ${{ needs.validate-ctan.outputs.texlive-year }})" >> notes.md
+          else
+            echo "Built from CTAN repository: \`$CTAN_REPO\`" >> notes.md
+          fi
 
       - name: Publish new release
         run: |