ci/treefmt: add gitMinimal for treefmt

treefmt needs git on the PATH for its default `tree-root` calculation,
`git rev-parse --show-toplevel`

(cherry picked from commit 8d94c496e9)

ci/treefmt.nix

@@ -4,6 +4,11 @@
   ...
 }:
 {
+  runtimeInputs = [
+    # tree-root uses `git rev-parse --show-toplevel`
+    pkgs.gitMinimal
+  ];
+
   settings = {
     # numtide/treefmt-nix defaults
     excludes = [