format: Fix failing due to no cache access

We change `$HOME` to something not writable, but newer treefmt fails if
it cannot write. We could use a tmpdir, but there’s not much point to a
cache that will be removed, as just add the flag to not use the cache.

format

@@ -32,4 +32,4 @@ git_root=$(git rev-parse --show-toplevel)
 git ls-files -z --cached --others --full-name -- "${files[@]}" |
     grep -z '\.nix$' |
     sed -z "s|^|$git_root/|" |
-    xargs -0 treefmt "${nixfmt_args[@]}"
+    xargs -0 treefmt --no-cache "${nixfmt_args[@]}"