ci: bump GitHub action runner

[SteveLauC]

What does this PR do

Bump GitHub action runner:

• macos-14 -> macos-latest

  Currently, macos-latest is macos-14, so no changes for now.

• ubuntu-20.04 -> ubuntu-latest

Update:

For cross test, we would still use Ubuntu 20.04 due to this issue: #2418.

Checklist:

• [x] I have read CONTRIBUTING.md
• [ ] I have written necessary tests and rustdoc comments
• [ ] A change log has been added if this PR modifies nix's API

[SteveLauC]

Created an issue #2418 for that CI failure, do you think we should ignore this test under QEMU? cc @asomers

[SteveLauC]

Created an issue #2418 for that CI failure, do you think we should ignore this test under QEMU? cc @asomers

I think we should still use Ubuntu 20.04 for that cross test as it won't harm anything until 20.04 is no longer available.

When 20.04 becomes deprecated, if this issue still exists, we will ignore it, or we can happily migrate to newer versions.