cli: show description of default value for --backend-local-temp-dir instead of value

[6543]

blocked by urfave/cli#2210 upstream fixed

before

at https://woodpecker-ci.org/docs/cli#exec :

now

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 21.29%. Comparing base (deadc0e) to head (230e092).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5656      +/-   ##
==========================================
- Coverage   21.31%   21.29%   -0.02%     
==========================================
  Files         420      420              
  Lines       38080    38080              
==========================================
- Hits         8116     8111       -5     
- Misses      29211    29216       +5     
  Partials      753      753              

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[woodpecker-bot]

Surge PR preview deployment was removed

[6543]

fixed bug upstream: urfave/cli-docs#17 ... let's see how long it takes to get merged