Fix build action

[apata]

The image we used to build the site was not pinned to exact version and the latest tag has moved since the last successful build end of Jun 2026.

This PR

• fixes the build action that happens on merge
• temporarily adds a workflow that runs on this PR to verify build action is fixed
• reapplies a local development helper that I thought was the source of the problem but wasn't

[apata]

@cnkk I've verified that the fix works using the temporary build on PR workflow: I downloaded the build artifact and served it, spot checking some pages. All looks well. Before merge, I will revert daf24aa, which was necessary only for this.