nixos-rebuild-ng: skip broken test for Nix < 2.18

The mock test for building with remote, but distinct target and build
host checks a bunch of internals (how many shell commands are executed,
which ones exactly) that are special cased on WITH_NIX_2_18, so the test
breaks when that is false. I've opted to simply skip the test for now
since the expected call list is very annoying to keep maintained and
updated for multiple versions.

This should fix NixOS configurations with config nix.package.version < 2.18.
To keep it that way, I've added nixos-rebuild-ng with some nixVersions
to passthru.tests that aren't likely to be removed in the short term.

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