Don't run preSwitchChecks during install

[phaer]

NixOS includes an option called system.preSwitchChecks since NixOS/nixpkgs@6e192c4.

As we are installing a new system here, we do not care too much about the systems state pre switching and therefore disable those checks.

This change is motivated by nix-community/srvos#591 (comment)

untested, but this might help @Kranzes?

[Kranzes]

I'd rather the workaround be done on the SrvOS side of things, if possible. Unless you think there will be more cases besides SrvOS for this?

[phaer]

Happy about alternative PRs! If you want to convince me, I'd need more of an argument as to why you think so.

Srvos already ships two mechanism by which to skip this check.

EXPECTED_HOSTNAME, which requires the target hostname that we don't have readily available.
And NIXOS_NO_CHECK which is used here as I couldn't come up with a preSwitchCheck that you would want to run during first install.

[Kranzes]

If you want to convince me, I'd need more of an argument as to why you think so.

You won. I don't have the energy LOL. I will test this PR later.

[zowoq]

PR to improve installer detection: nix-community/srvos#601.

Probably a good idea to merge this PR anyway.