Enable aarch64-linux but disable it in hydra

[carbolymer]

Changelog

- description: |
    Enable aarch64-linux but disable it in hydra
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
   - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...

Context

We don't have aarch64-linux native builders for now. This PR simply enables this architecture in flake, but it does not build it in hydra.

Fixes #805.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[palas]

Backup: https://github.com/IntersectMBO/cardano-cli/tree/backup/mgalazyn/chore/enable-aarch64-linux

[palas]

FYI: I have rebased your branch because we have done changes to the formatting. I have made a copy of the unrebased branch that you can find in my previous comment in this PR.