saveWorkflowsFromPipelineBuilder: check for pre-loaded workflows

[6543]

based on #3967

[woodpecker-bot]

Surge PR preview deployment succeeded. View it at https://woodpecker-ci-woodpecker-pr-6474.surge.sh

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 41.62%. Comparing base (e4dfbf8) to head (0a8fa83).

Files with missing lines	Patch %	Lines
server/pipeline/items.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6474   +/-   ##
=======================================
  Coverage   41.61%   41.62%           
=======================================
  Files         432      432           
  Lines       28751    28752    +1     
=======================================
+ Hits        11964    11967    +3     
+ Misses      15715    15712    -3     
- Partials     1072     1073    +1     

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