ci: Run cargo fmt for the entire workspace

Currently only `src/` is checked. Add a run that covers everything else.

This does mean files in `src/` get formatted twice, but `cargo fmt`
doesn't support an `--exclude` option.

(backport <#4454>)
(cherry picked from commit 1ca8b36)

Author: tgross35

ci/style.sh

@@ -59,6 +59,10 @@ done < "$tmpfile"
 
 rm "$tmpfile"
 
+# Run once from workspace root to get everything that wasn't handled as an
+# individual file.
+cargo fmt
+
 if shellcheck --version ; then
     find . -name '*.sh' -print0 | xargs -0 shellcheck
 else