nixosTests.installer.switchToFlake: It is probably really stupid

We may want to clear NIX_PATH when channels are disabled, or maybe
it has to be a separate option.
This is just very frustrating to me.

Changed files
+1
nixos
+1
nixos/tests/installer.nix
···
# - the "backdoor" shell is not a proper session and does not have `NIX_PATH=""` set
# - seeing no nix path settings at all, Nix loads its hardcoded default value,
# which is unfortunately non-empty
+
# Or maybe it's the new default NIX_PATH?? :(
# with subtest("builtins.nixPath is now empty"):
# target.succeed("""
# (