ci(pre-commit): show diff on failure

Mainly for shfmt, which writes its changes in place. shfmt has the `-d`
option which would output the diff for us, but showing the diff on
pre-commit level is preferable in CI in the first place. And the diff
emitted by it has colors, unlike the shfmt one.

Author: scop

.github/workflows/ci.yaml

@@ -67,7 +67,7 @@ jobs:
           # Other pre-commit checks
 
           set +e
-          pre-commit run --color=always --all-files
+          pre-commit run --color=always --all-files --show-diff-on-failure
           ((rc+=$?))
           set -e