The number of jobs should scale with the runner CPU count, up to a
point. The bottleneck becomes network / disk I/O at very large numbers,
and 8 seems like a reasonable maximum based on my experience at this
time.

* As https://nodejs.org/api/os.html#oscpus says:
  os.cpus().length should not be used to calculate the amount of parallelism available to an application. Use os.availableParallelism() for this purpose.

* Fixes #827

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

* See discussion in rake-compiler/rake-compiler#247
  it is messy if that's not the case and also creates inconsistencies
  based on which Java tool/executable is used.