nixos-rebuild: Added a version check

[crazazy]

Motivation for this change

Before this patch, if /etc/nixos/flake.nix existed, nixos-rebuild would
try to use flakes even if you had an unsupported version of nix. we now
do an additional version check to see if nix can use flakes in the first
place.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[Synthetica9]

This is gonna cause a problem for nix 2.12.1-3 🤔

[r-rmcgibbo]

Result of nixpkgs-review pr 117707 at 1594d24 run on x86_64-linux 1

1 package built successfully:

• nixos-rebuild

[crazazy]

This is gonna cause a problem for nix 2.12.1-3 thinking

As far as I am aware, after this we should be moving to 3.0
thought I might want to add another dot to the regex so that we dont accidentally check for a minor+hotfix combination yeah...

[stale]

I marked this as stale due to inactivity. → More info

[wamserma]

I think this should not be a problem any more, as relevant Nix versions (2.1,2.2,2.3) have largely been retired and a rewrite of nixos-rebuild has landed in #354029.