Chore: Use F5 artifactory GOPROXY and self-hosted runners

[ciarams87]

Proposed changes

• Use F5 Artifactory goproxy for all CI runs that do not originate from forks
• Use self hosted runners for jobs that produce artifacts (so on builds from main and tags)

Testing: https://github.com/nginx/nginx-gateway-fabric/actions/runs/16878757544 (the relevant jobs are the build binary and images jobs, the failed test jobs can be ignored - I had to make temp changes to test the runners work correctly)

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

NONE

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.91%. Comparing base (00112a4) to head (07e7192).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3718      +/-   ##
==========================================
- Coverage   86.95%   86.91%   -0.04%     
==========================================
  Files         128      128              
  Lines       15962    15962              
  Branches       62       62              
==========================================
- Hits        13879    13873       -6     
- Misses       1917     1921       +4     
- Partials      166      168       +2     

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