I've removed that deprecated option, and used the new correct syntax for the tag index.
Builds work again.

Unfortunately,when I've rerun build (when I could), it still fails.

@jxu @mhayter
The build works.

Why rerunning some builds doesn't work automatically, is due to technical decisions by Github.

The build step doesn't automatically use the code from the main branch. It actually uses a combination (Git Merge) of the PR and the main branch to the time when the pull request was opened.
So for all the PRs that were opened before my fix last week, the builds use still the code of the old main branch.

That sounds a bit odd. Especially when you come from other Code Versioning / DevOps platforms. E.g. I currently work a lot with Azure DevOps, and there the build always runs on the merge between the most recent target branch and your PR branch.
But I guess Github made the decision to use the creation time as a merge base, because lots of repositories have lots of PRs.
And you don't want to rerun all builds (costs saving), just because the main branch changed.

Anyway. There are two easy solutions.
You can just merge the current main branch into the PR:

Or alternatively close and reopen the PR (which is nicer to the creator of the PR, as they will not have additional commits in their Fork/Branch):