ci: also run jobs on a schedule

We have been bitten before by `codespell` or `cargo clippy` adding new
checks in times of low activity on the repo, resulting in the next
pull requests to fail.

Make sure we notice such changes early by running the jobs on a schedule
every then and now.
The scheduled time of 20:48 UTC every thursdays was chosen by fair
dice roll.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>

.github/workflows/codespell.yaml

@@ -5,6 +5,8 @@ on:
   push:
     branches:
       - main
+  schedule:
+    cron: 48 20 * * 4
 
 jobs:
   codespell:

.github/workflows/tacd-daemon.yaml

@@ -7,6 +7,8 @@ on:
   push:
     branches:
       - main
+  schedule:
+    cron: 48 20 * * 4
 
 env:
   # Make sure cargo commands not only fail on hard errors but also on warnings

.github/workflows/tacd-webui.yaml

@@ -8,6 +8,8 @@ on:
   push:
     branches:
       - main
+  schedule:
+    cron: 48 20 * * 4
 
 env:
   # Generating sourcemaps fails with file not found in the autolinker