Include runner arch in CI cache key

[cdce8p]

Proposed change

Recently Github added the ubuntu-24.04-arm runners. Include runner.arch (either X64 or ARM64) in all cache keys to prevent accidentally restoring an incompatible cache if at some point we switch to using the arm runners.

E.g. mypy recently switched their pytest jobs over as those ran a bit faster than on X64. This should be properly evaluated for our CI jobs first before making the switch as well.

After the initial rebuild of the caches, this change itself is a no-op, similar to bumping the cache version.

https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/accessing-contextual-information-about-workflow-runs#runner-context

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