CI: the self-test misses the stale-entry notice and the CR escape, and annotates what it quotes

[vharseko]

The third review round of #28 (review 5389582272), on code that reached master through #38:

1. The self-test of check.sh never makes it print a "No longer found" notice. It runs with empty known lists and with known == build, so fixed is always empty. comm -13 → comm -12, fixed= or ::notice → ::debug in check.sh keep the Build check green, though a known entry that is still present would then be announced as gone, and a stale one would not.
2. No test feeds a CR, so the %0D escape of the annotation text in check.sh can be deleted without any test failing.
3. When a self-test fails, its message echoes the output of check.sh / head-meta.sh to stderr. The runner reads workflow commands on stderr too, so every ::error line after the first becomes an annotation of the pull request, naming a page and a link that do not exist. This happens in three places of .github/workflows/build.yml: two in "Self-test the baseline check" and one in "Self-test the head meta check".

Expected: the self-test fails when the notice of a stale entry or the CR escape breaks, and a failing self-test does not annotate the pull request with the lines it quotes.