release.nix: fix eval with aarch64-, but not x86_64-darwin supported
release.nix: fix eval with aarch64-, but not x86_64-darwin supported

We emit a few jobs conditionally on supportDarwin which only checked for
x86_64-darwin in the past. This change makes it more modular by
transforming it into an attribute set which holds the two darwin
arches. Jobs needing aarch64-darwin or x86_64-darwin are now only
emitted if their respective platform is actually in supportedSystems.

This issue was discovered because the staging-next-21.11 jobset had
commented out x86_64-darwin (presumably due to a build load issue).

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