Move Linux ARM64 "build" test from CircleCI to GitHub Actions

[SWilson4]

This PR moves the CircleCI ARM64 Linux "build" test over to GitHub Actions, using the new oqs-arm64 runner.

It also fixes some GitHub syntax so that jobs are skipped when we expect them to be.

Makes progress on #1795.

• Does this PR change the input/output behaviour of a cryptographic algorithm (i.e., does it change known answer test values)? (If so, a version bump will be required from x.y.z to x.(y+1).0.)
• Does this PR change the list of algorithms available -- either adding, removing, or renaming? Does this PR otherwise change an API? (If so, PRs in fully supported downstream projects dependent on these, i.e., oqs-provider will also need to be ready for review and merge by the time this is merged.)