[ci] Don't run prdoc and lables GHA on master (#3257)

PR adds condition to ignore master branch for prdoc and labels GHA.

This option doesn't work because all PRs are for master thus the actions
won't start:
```yml
on:
  pull_request:
    branches-ignore:
      - master
```

This option doesn't work because actions don't see the PR number and
[break](https://github.com/paritytech/polkadot-sdk/actions/runs/7827272667/job/21354764953):
```yml
on:
  push:
    branches-ignore:
      - master
```

cc https://github.com/paritytech/ci_cd/issues/940
cc https://github.com/paritytech/polkadot-sdk/issues/3240