Enable CPU tests on default ARM build

[hugovk]

Some CPU-heavy tests are marked with the cpu resource and we don't run them on GitHub Actions because they're slow. This means we sometimes don't catch cpu failures until after they've been merged and run on the buildbots, which is a slow feedback cycle.

But how slow are the cpu tests?

Here's a recent GHA run, without any cpu (🔒 Datadog):

The whole thing is around 31m, the slowest jobs are free-threading Windows arm64 (29.5m) and x64 (24.5m), with four others at 20-22m.

One of the fastest is the default ubuntu-24.04-arm at 9.5m.

(The ubuntu-24.04 (x64) one is 13m.)

If we add cpu resources to just this job, it only takes an extra ~3 minutes (total around 12.5 mins), still ranking it among one of the fastest jobs on the CI:

https://github.com/python/cpython/actions/runs/21947082050/job/63387452394?pr=144743

[hugovk]

Comparing with and without caches:

	main	PR
uncached	10m 24s + 9m 52s	13m 36s + 14m 47s
cached	9m 13s + 8m 41s + 8m 54s	12m 37s + 13m 29s + 13m 45s