nixos/release-small: Add aarch64-linux to supportedSystems
nixos/release-small: Add aarch64-linux to supportedSystems

The aarch64-linux builders on hydra have had a good track in the last
year or so and I think it's a good idea to include them in the default
-small jobset.

This happens in preparation of improving the distribution of the
installer ISOs for aarch64-linux systems and advertise them more
prominently on the homepage.

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