The `group` expression ensures that the cancel-in-progress
behavior is to only cancel if a new commit is pushed to the PR for
which the job is running, not other PRs.

This is a fairly standard snippet, used also in CI jobs for NumPy
and other projects.

This is a follow-up to gh-4271. At the moment, when a contributor
pushes the latest `develop` to their own branch to bring their own
fork in sync with `main`, or if they push another branch, this triggers
30 CI jobs to run. Most will complete silently and only burn CPU
time unnecessarily. If there's a failure, this may result in unexpected
failure notifications. And the AWS Graviton3 run won't complete at all
and time out, since the Cirun hook will only work when triggered from
the main repo.

Signed-off-by: Alex <aleksandrosansan@gmail.com>

CI (MSYS2): Configure with `-DCMAKE_BUILD_TYPE=Release`.

Add cross compile tests without running checks. Currently
only mips64el, riscv64, mipsel, alpha is wired up.
Just help us make sure those less popular CPUs are not
messed up by changes.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

Re-organize build matrix for Ubuntu and MacOS runners.
Don't start runners that don't do anything.
Run tests.