CI: Support customizing CONTRIBUTORS in new contributor PRs (#2128)

CI: Support customizing CONTRIBUTORS in new contributor PRs (#2128)

When our source-maintenance.sh recognizes a magic phrase in a commit
message, it treats that commit as authoritative for CONTRIBUTORS file.
That feature allows us to customize CONTRIBUTORS by adding a magic
phrase to the commit message, preventing unwanted automated
collectAuthors() actions for earlier commits. Such commits work well
enough when CONTRIBUTORS is customized by an already known Squid
developer in a dedicated PR.

The same feature cannot work for PRs created by new developers because
when quick.yaml tests a PR, the first commit visible to collectAuthors()
is not a PR branch commit that the author could, with some considerable
trouble, customize, but a so called GitHub PR "merge commit" that GitHub
automatically creates with PR creator as the author. That merge commit
message does not contain PR description and never has our magic phrase.
That merge commit author details are wrong when PR creators want to use
information that differs from their public GitHub account info.

With this change, collectAuthors() consults PR description (when testing
PRs), allowing CONTRIBUTORS customization without any extra efforts:
Adding the magic phrase to PR description has to be done anyway.