🐛 Make checks.nixfmt work from everywhere

If $nixFiles were expressed as relative to the root of the project,
nixfmt would fail when ran from a sub directory. Now cd to the root
first which will work for both relative and absolute paths.

Since nix (and thus we) have not committed to using flakes this instead
uses git to find the root of the project.

CHANGELOG.md

@@ -6,6 +6,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- `.#checks.nixfmt` can be run from everywhere when nixFiles are relative to project.
+
 ## [10.0.0] - 2024-02-06
 
 ### Added

ci/nixfmt

@@ -13,6 +13,8 @@ if [[ $* =~ (^|[[:space:]])+--fix([[:space:]]|$)+ ]]; then
     check=""
 fi
 
+cd "$(git rev-parse --show-toplevel)"
+
 if [ $# -eq 0 ] || [ -z "$check" ]; then
     exitCode=0
     files=()