[Auto publication] Force job serialization

One job runs for each of the 3 documents in the repository. Each of them
updates the "gh-pages" branch. Problem is that these jobs are typically run in
parallel, leading to "not up to date" issues when "git push" commands are run.

For an example of such a problem, see:
https://github.com/w3c/webcodecs/runs/2462176606

Also see discussion in:
w3c/spec-prod#58

This update tells GitHub to serialize the jobs. Since the jobs are relatively
independent otherwise, it also tells GitHub to run them to completion even
when one of them fails.

.github/workflows/pr-push.yml

@@ -19,6 +19,8 @@ jobs:
   main:
     runs-on: ubuntu-20.04
     strategy:
+      fail-fast: false
+      max-parallel: 1
       matrix:
         include:
           - source: index.src.html