scripts/with-go-mod.sh: use symlink instead of -modfile

[thaJeztah]

While the "-modfile=vendor.mod" is slightly more correct, using a symlink allows for most of the go tools to work "as usual", just without an acual go.mod being committed in the repository.

- Human readable description for the release notes

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 55.15%. Comparing base (518ba2b) to head (4464134).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6039      +/-   ##
==========================================
+ Coverage   55.13%   55.15%   +0.01%     
==========================================
  Files         358      358              
  Lines       30023    30023              
==========================================
+ Hits        16553    16558       +5     
+ Misses      12523    12519       -4     
+ Partials      947      946       -1     

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