linkcheck should be performed on a merge of master into PR

[DickJC123]

Description

When the unittests are run, the built repo under test is not the PR's HEAD commit, it's a merge of master into the PR. One nice thing about this is that if the PR is accepted, the result of merging the accepted PR into master is exactly what was tested. Also, if a different PR is merged into master that fixes test failures, it immediately helps all PR's in development- they don't need to merge the new master into the PR to pick up the fix. Sadly, there's a dark side to this: an accepted PR that introduces a flakey test will hamper in-development PR's immediately.

We recently had a linkcheck failure fixed on master by PR #20879, but the linkcheck failure was not eliminated for in-development PRs. Only after remerging master into the PR did the failure disappear. Shouldn't the same approach of merging master into the PR be used with linkcheck, as it is with the unittests?