Maintain same ownership when formatting

[infinisil]

Fixes #73

$ cabal install --installdir=$PWD/bin
Wrote tarball sdist to
/home/tweagysil/src/nixfmt/dist-newstyle/sdist/nixfmt-0.6.0.tar.gz
Resolving dependencies...
Symlinking 'nixfmt' to '/home/tweagysil/src/nixfmt/bin/nixfmt'

$ echo '1  +  1' > a.nix
$ sudo chown root:root a.nix

$ bin/nixfmt a.nix
nixfmt: ./.a.nix357406-0.atomic: setOwnerAndGroup: permission denied (Operation not permitted)
$ cat a.nix
1  +  1
$ stat -c "%U:%G" a.nix
root:root

$ sudo bin/nixfmt a.nix
$ cat a.nix
1 + 1
$ stat -c "%U:%G" a.nix
root:root

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This work is sponsored by Antithesis ✨

[github-actions]

Nixpkgs diff

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/formatting-team-meeting-2024-09-03/51584/1