fix: refactor publish pipeline (#13)

## Description

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relates to #1234

## Checklist

- [ ] Issue was linked above
- [ ] Code format was applied: `make fmt`
- [ ] Examples were added / adjusted (see `examples/` directory)
- [x] Docs are up-to-date: `make generate-docs` (will be checked by CI)
- [ ] Unit tests got implemented or updated
- [ ] Acceptance tests got implemented or updated (see e.g. [here](f5f99d1709/stackit/internal/services/dns/dns_acc_test.go))
- [x] Unit tests are passing: `make test` (will be checked by CI)
- [x] No linter issues: `make lint` (will be checked by CI)

Reviewed-on: #13
Co-authored-by: Marcel S. Henselin <marcel.henselin@stackit.cloud>
Co-committed-by: Marcel S. Henselin <marcel.henselin@stackit.cloud>

.github/workflows/publish.yaml

@@ -110,7 +110,7 @@ jobs:
 
       - name: Prepare key file
         run: |
-          echo $(echo ${{ secrets.KEY_FILE_B64 }} | base64 -d) >public_key.pem
+          echo "${{ secrets.PUBLIC_KEY_PEM }}" >public_key.pem
 
       - name: Prepare provider directory structure
         run: |