Improvements to the Emscripten buildbot

* Use the build script from its new `Platform/emscripten/__main__.py` location
* Use `python3 Platform/emscripten run` to avoid hard coding build output path
* Use new `EMSDK_CACHE` environment variable to allow build script to manage
  Emscripten installations
* Allow build script to manage node installations

python/cpython#146156 has to land first

Author: hoodmane

master/custom/factories.py

@@ -1357,48 +1357,42 @@ class EmscriptenBuild(BaseBuild):
 
     def setup(self, **kwargs):
         compile_environ = {
-            "PATH": os.pathsep.join([
-                "/home/emscripten/emsdk",
-                "/home/emscripten/emsdk/upstream/emscripten",
-                "/home/emscripten/.local/bin",
-                "/usr/local/bin",
-                "/usr/bin",
-                "/bin",
-            ]),
-            "EMSDK": "/home/emscripten/emsdk",
-            "PYTHON_NODE_VERSION": "24",
+            "EMSDK_CACHE": "/home/emscripten/emsdk-versions",
         }
 
         self.addSteps([
             Configure(
                 name="Configure build Python",
-                command=["python3", "Tools/wasm/emscripten", "configure-build-python"],
+                command=["python3", "Platform/emscripten", "configure-build-python"],
                 env=compile_environ,
             ),
             Compile(
                 name="Compile build Python",
-                command=["python3", "Tools/wasm/emscripten", "make-build-python"],
+                command=["python3", "Platform/emscripten", "make-build-python"],
                 env=compile_environ,
             ),
             Compile(
                 name="Compile host libFFI",
-                command=["python3", "Tools/wasm/emscripten", "make-libffi"],
+                command=["python3", "Platform/emscripten", "make-libffi"],
                 env=compile_environ,
             ),
             Configure(
                 name="Configure host Python",
-                command=["python3", "Tools/wasm/emscripten", "configure-host"],
+                command=["python3", "Platform/emscripten", "configure-host"],
                 env=compile_environ,
             ),
             Compile(
                 name="Compile host Python",
-                command=["python3", "Tools/wasm/emscripten", "make-host"],
+                command=["python3", "Platform/emscripten", "make-host"],
                 env=compile_environ,
             ),
             Test(
                 name="Node full test suite",
                 command=[
-                    "cross-build/wasm32-emscripten/build/python/python.sh",
+                    "python3",
+                    "Platform/emscripten",
+                    "run",
+                    "--",
                     "-m", "test",
                     "-v",
                     "-uall",
@@ -1412,14 +1406,14 @@ def setup(self, **kwargs):
             Test(
                 name="PyRepl in Chrome smoke test",
                 command=[
-                    "Tools/wasm/emscripten/browser_test/run_test.sh",
+                    "Platform/emscripten/browser_test/run_test.sh",
                 ],
                 env=compile_environ,
                 timeout=step_timeout(self.test_timeout),
             ),
             Clean(
                 name="Clean the builds",
-                command=["python3", "Tools/wasm/emscripten", "clean"],
+                command=["python3", "Platform/emscripten", "clean"],
                 env=compile_environ,
             )
         ])