xmllint always marks all files as changed

**Describe the bug** Any time any file is checked for formatting issues by `xmllint`, it is considered to be changed, so `fail-on-change` causes `nix fmt` to always fail. **To Reproduce** Steps to reproduce the behavior: 1. Set `programs.xmllint.enable = true;` 2. Set `settings.fail-on-change = true;` 3. Add a formatted XML file 4. Run `touch *.xml` 5. Run `nix fmt` At this point, you'll get a "ERRO file has changed" error message and exit code 1. **Expected behavior** If the XML files are already formatted, nothing should be printed and the exit code should be zero. **System information** - system: `"x86_64-linux"` - host os: `Linux 6.12.63, NixOS, 26.05 (Yarara), 26.05.20251228.c0b0e0f` - multi-user?: `yes` - sandbox: `yes` - version: `nix-env (Nix) 2.31.2+1` - channels(root): `"home-manager, nixos, nixos-hardware, nixos-stable-24.05"` - nixpkgs: `/nix/store/xzl5dhddgjxp2p18h5qpkhvlkp8128j1-source` **Additional context** The following hack can be used to work around this: 1. Disable `fail-on-change` 2. Manipulate the treefmt cache to think XML files don't need changes by running `nix fmt` 3. Re-enable `fail-on-change` Subsequent runs will not treat any XML files as changed unless `touch`ed or otherwise changed. --- It looks like `xmllint` changes the time stamp of every single file, presumably by unconditionally overwriting each of them. This should be avoided - the time stamp should only change if the file was actually changed.