GHA/checksrc: check GHA rules with zizmor

The pedantic level is experimental. If it causes issues, we may just
disable it alongside the ignore comments.

Also:
- silence error:
  ```
   INFO audit: zizmor: completed label.yml
  error[dangerous-triggers]: use of fundamentally insecure workflow trigger
    --> label.yml:13:1
     |
  13 | 'on': [pull_request_target]
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^ pull_request_target is almost always used insecurely
     |
     = note: audit confidence -> Medium
  ```
- fix pedantic warning:
  ```
   INFO audit: zizmor: completed label.yml
  warning[excessive-permissions]: overly broad permissions
    --> label.yml:1:1
  ...  |
  24 | |         with:
  25 | |           repo-token: '${{ secrets.GITHUB_TOKEN }}'
     | |____________________________________________________- default permissions used due to no permissions: block
     |
     = note: audit confidence -> Medium
  ```
- silence `template-injection` false positives like:
  ```
  - note: ${{ matrix.build.torture && 'test-torture' || 'test-ci' }} may expand into attacker-controllable code
  - note: ${{ contains(matrix.build.install_steps, 'pytest') && 'caddy httpd vsftpd' || '' }} may expand into attacker-controllable code
  ```
  It doesn't seem like these could be controlled by an attacker.
  Let me know if I'm missing something.

Closes #17278

.github/workflows/label.yml

@@ -10,11 +10,12 @@
 # https://github.com/actions/labeler
 
 name: Labeler
-'on': [pull_request_target]
+'on': [pull_request_target]  # zizmor: ignore[dangerous-triggers]
+
+permissions: {}
 
 jobs:
   label:
-
     runs-on: ubuntu-latest
     permissions:
       contents: read