CI: use smaller machines in PR runs
mingw-check job-linux-16c -> job-linux-4c
~job-linux-4c 20 min in auto job
~job-linux-16c 13 min in pr job
with current pr regressed to almost 21 min, it's ok.
mingw-check-tidy job-linux-16c -> job-linux-4c small enough, so reduce to minimal
~ job-linux-16c 3 min
with current pr regressed to almost 5 min, it's ok.
x86_64-gnu-tools job-linux-16c this is top job by time in PR, so don't touch it
~ job-linux-8c 1.30 hour in auto job
~ job-linux-16c 1 hour in pr job (affected by #114613, actual time ~ 30 min)
x86_64-gnu-llvm-15 job-linux-16c don't change too
~ job-linux-8c 1.30 hour in auto job
~ job-linux-16c 30 min in pr job
Noticed while working on https://github.com/rust-lang/rust/pull/114621, so current time affected by always rebuilded docker images (but pr images always rebuilded before too, so nvm)
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