nixos/release.nix: Introduce callSubTestsOnTheseSystems

The existing callSubTests seems to already have special-cased code to
allow enabling subtests on a single specific system by looking at the
`system` attribute in the test arguments. Replace it with a new version
similar to the callTestOnTheseSystems because:

- It's consistent with the existing functions for creating
  system-specific tests (though admittedly, the callSubTests special
  case for `system` predates them)
- This approach allows limiting to multiple system types, the previous
  one inherently allows only one system type.
- This also fixes the problem that if you pass in e.g.
  supportedSystems = [ "aarch64-linux" ], you end up with a
  tests.chromium job that silently runs on x86_64-linux.
- Finally, this causes renames of the jobs like:
  tests.chromium -> tests.chromium.x86_64-linux to be consistent with
  the rest of the tests.

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