nixos-rebuild: add edit command

[LnL7]

Motivation for this change

Inspired by LnL7/nix-darwin#94.

As a user I don't want to remember where the configuration exists. This
adds a command that will point the $EDITOR to the file.

/cc @zimbatm

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• 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 nox --run "nox-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)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.

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[zimbatm]

thanks for doing this

[infinisil]

I'm no bash guru, but I thought exec would relinquish all control, so this line won't ever be executed anyways (and won't be needed), or does it serve any purpose?

[LnL7]

I've seen this pattern in multiple languages. I'm not sure if there is some weird case that this actually handles or if it's just for clarity.

[xeji]

so this line won't ever be executed anyways

If the exec call itself fails for some reason (e.g. an EDITOR that is not available), the script would continue and the exit 1 forces an exit. So in general, this makes sense. But in this particular case it's not necessary because set -e at the beginning forces global exit-on-error anyway. But it doesn't hurt either 😄

[infinisil]

Oh, this should really have a release note though

[LnL7]

I'll make another pr for the changelog and docs.

[ElvishJerricco]

Don't some flags (e.g. --option, -I) need to be passed to the nix-instantiate call?