nixos/testing: only create `nixos-run-vms` for `nixos-build-vms(8)`
nixos/testing: only create `nixos-run-vms` for `nixos-build-vms(8)`

This is the case when the test-script is empty. `nixos-build-vms(8)` is
primarily supposed to be used as tool to test changes or to reproduce
bugs (IMHO) where "just spinning up a few VMs" is the primary use-case.

In the ongoing discussion about these changes[1] it was suggested to
only expose it when needed (i.e. in the case I described above) to keep
the API surface as slim as possible.

[1] https://github.com/NixOS/nixpkgs/pull/133675#discussion_r688112485

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