Fix sync recipes and add base justfile

Both the justfile and the pre-commit configuration for the `pr-check`
sync were broken:
* justfiles run recipes one line at a time in a fresh shell, so the venv
  activation was not working
* the pre-commit config was relying on an installed `ruamel.yaml`
  pakcage, but the default one installable via `apt` on Ubuntu 24.04 is
  old and generates different output (with formatting differences).

Now:
* the venv dance is put in a separate bash script
* both just and pre-commit will use that same script, so both problems
  will be fixed

As a bonus, a root `justfile` is added exposing the `update-pr-checks`
recipes plus a `build` one. Running `just` without arguments will also
now call the default `sync` recipes that will call both of the above.

.pre-commit-config.yaml

@@ -16,5 +16,5 @@ repos:
         name: Synchronize PR check workflows
         files: ^.github/workflows/__.*\.yml$|^pr-checks
         language: system
-        entry: python3 pr-checks/sync.py
+        entry: pr-checks/sync.sh
         pass_filenames: false