`@ofborg test installer` did not run installer tests, yet succeeded

[Robert Hensing]

See NixOS/nixpkgs#177672 (comment)

[Naïm Favier]

Duplicate of #504, the workaround is @ofborg build nixosTests.installer.

[Cole Helbling]

Since #630, we've switched to using nixosTests rather than the nixos/release.nix's tests :)