Chore: fix helm and NFR tests in the pipeline

[ciarams87]

Proposed changes

Problem: The cert-generator job was failing with:

Error: failed to create containerd task: OCI runtime create failed: runc create failed: 
unable to start container process: exec: "/usr/bin/gateway": permission denied

because GitHub Actions' actions/download-artifact does not preserve file permissions. When the binary artifacts are uploaded and then downloaded, they lose the execute bit.
The Flow

Binary job → goreleaser builds binaries with 755 permissions → uploads to artifacts
Artifacts → stored without preserving execute permissions
Helm test/ NFR job → downloads artifacts → binaries now have 644 permissions (no execute)
Docker build → COPY dist/... preserves the 644 permissions
Container runtime → tries to execute /usr/bin/gateway → permission denied

Solution: Added --chmod=0755 to all COPY commands in build/Dockerfile that copy the gateway binary
This ensures the binary always has execute permissions in the final image, regardless of the source file permissions.

Also fixed a separate issue that prevented make build TARGET=container from working (wrong module path) and fixed the conditions for the nightly schedule to use the correct runner

Testing: https://github.com/nginx/nginx-gateway-fabric/actions/runs/18586202163/job/52991407973?pr=4096

Please focus on (optional): If you any specific areas where you would like reviewers to focus their attention or provide
specific feedback, add them here.

Closes #ISSUE

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.69%. Comparing base (7374a97) to head (ed3a168).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4096      +/-   ##
==========================================
- Coverage   86.71%   86.69%   -0.02%     
==========================================
  Files         128      128              
  Lines       16758    16758              
  Branches       62       62              
==========================================
- Hits        14531    14529       -2     
- Misses       2042     2043       +1     
- Partials      185      186       +1     

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