nixos/hydra: fix test

We ship `https://cache.nixos.org` as binary cache by default which
automatically substitutes the test derivation used inside the Hydra
test. However it needs to be built locally to confirm that
`hydra-queue-runner` works properly.

Also inherited the platform name for the test derivation from `system`
to ensure that the build can be tested on each supported platform.

ZHF #68361