nixos/release-combined.nix: fix tested/supportedSystems

This properly supports the `supportedSystems` and
`limitedSupportedSystems` arguments of `release-combined.nix`.
Previously, evaluation would fail if `x86_64-linux` was not part either
of those, since the tested job always referenced the `x86_64-linux`
nixos tests (which won't exist in an aarch64-only eval).

Since the hydra configuration for the jobset`trunk-combined` has both
`aarch64-linux` and `x86_64-linux` as supported systems, this will make
aarch64 be part of the tested job on that jobset.