Possible GitHub Pages update conflicts in multi-specs repos

[tidoust]

The matrix setting effectively duplicates the job. These jobs seem to run in parallel. This can create conflicts when they check out the GitHub Pages branch and try to update it at the same time.

That typically happened with last commit to WebCodecs:
https://github.com/w3c/webcodecs/runs/2462176606

git push --force-with-lease origin "gh-pages"
    To https://github.com/w3c/webcodecs.git/
     ! [remote rejected] gh-pages -> gh-pages (cannot lock ref 'refs/heads/gh-pages': is at a369eebbf6d854fd3815c6bf26afc9e577f47d26 but expected e9dfdfa373befa8cff557a6fd66221ff077ef686)
    error: failed to push some refs to 'https://github.com/w3c/webcodecs.git/'

I see that there exists a strategy.max-parallel setting that can probably be set to 1 to serialize the jobs. I don't know if there's a way to do that in spec-prod directly instead of requiring that setting to be set in the YML file.

[sidvishnoi]

strategy.max-parallel setting would be a fine way if you're ok with a slow CI. It would need to be set in the workflow (the action can't do that).

I was hoping that git pull just before git push might reduce the chances of clash. I will add some code to retry the push if it fails due to a clash.