Annotate javac warnings from the build job only

[abashev]

setup-java's problem matcher turns javac's warnings, Error Prone's among them, into annotations on the diff, and it is on by default. It was on in every job that compiles the same sources: build, the three newer JDKs, the five native platforms and both CodeQL analyses. Windows never annotated, because the matcher's pattern takes no path with a colon in it.

GitHub keeps only the first ten warnings of a step. The build prints 188 of them, and as it runs in parallel, each job prints them in its own order and keeps its own ten. That is why on #98 one warning shows up to ten times, the one next to it once, and most not at all. None of them come from #98: it adds no warning and removes one.

What changes

• build keeps the matcher, now explicitly and with a comment saying why.
• Every other setup-java step that runs on a pull request, in ci.yml and codeql-analysis.yml, passes problem-matcher: false. release.yml and dependency-submission.yml never run on one and are left as they are.

Each warning now shows at most once. Still only ten show, the first the parallel build prints, so which ones appear depends on the build order rather than on the PR until the backlog is worked off: 107 of the 188 are NullAway's.

Checked

• Both workflows parse (act --list), and every setup-java step has the setting above.
• This PR's own CI run is the real check: the warning annotations should all come from build, once each.