treewide: use strict mode for nixfmt

[0xda157]

inspired by https://github.com/nix-community/stylix/pull/1976#discussion_r2539186937

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• [x] I certify that I have the right to submit this contribution under the MIT license
• [x] Commit messages adhere to Stylix commit conventions
• [ ] Theming changes adhere to the Stylix style guide
• [ ] Changes have been tested locally
• [ ] Changes have been tested in testbeds
• [ ] Each commit in this PR is suitable for backport to the current stable branch