CI: run checks on a schedule and allow workflow_dispatch

Running checks on a schedule makes sense for our `ruff` lints and
our `codespell` spell checks, because these tools learn new lints/typo
fixes every then and now.
Without periodic job runs these new lints/typo fixes will only surface
when jobs for the next pull request fail for unrelated reasons.

The execution time of the scheduled job is chosen somewhat arbitrarily.

While at it also enable workflow_dispatch as a trigger, which can be used
to run the jobs manually.

Signed-off-by: Leonard Göhrs <[email protected]>

.github/workflows/check-and-publish.yaml

@@ -1,6 +1,11 @@
 name: Check and Publish
 
-on: [push, pull_request]
+on:
+  pull_request:
+  push:
+  schedule:
+    - cron: '22 15 * * 3'
+  workflow_dispatch:
 
 jobs:
   codespell: