Fix workflow serialize to omit skip_clone if false

[6543]

as skip_clone is false by default we can omit it.

this also prepares the test to use the modern workflow RunsOn to use the when filter.

fixes https://codeberg.org/lafriks/woodpecker-pipeline-transform/issues/35

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 33.30%. Comparing base (f842ce2) to head (b52ec43).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6319   +/-   ##
=======================================
  Coverage   33.30%   33.30%           
=======================================
  Files         420      420           
  Lines       28347    28347           
=======================================
  Hits         9440     9440           
  Misses      18029    18029           
  Partials      878      878           

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[6543]

@lafriks -> https://codeberg.org/lafriks/woodpecker-pipeline-transform/pulls/38