CI: misc cleanup & fixes (#8559)

## Problem
A bunch of small fixes and improvements for CI, that are too small to
have a separate PR for them

## Summary of changes
- CI(build-and-test): fix parenthesis
- CI(actionlint): fix path to workflow file
- CI: remove default args from actions/checkout
- CI: remove `gen3` label, using a couple `self-hosted` +
`small{,-arm64}`/`large{,-arm64}` is enough
- CI: prettify Slack messages, hide links behind text messages
- C(build-and-test): add more dependencies to `conclusion` job

.github/actions/run-python-test-set/action.yml

@@ -83,7 +83,6 @@ runs:
       uses: actions/checkout@v4
       with:
         submodules: true
-        fetch-depth: 1
 
     - name: Cache poetry deps
       uses: actions/cache@v4