fix: need to create key file from secret (#5)

## 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: #5
Co-authored-by: Marcel S. Henselin <marcel.henselin@stackit.cloud>
Co-committed-by: Marcel S. Henselin <marcel.henselin@stackit.cloud>

.github/workflows/publish.yaml

@@ -78,7 +78,8 @@ jobs:
 
       - name: Import GPG key
         run: |
-          gpg --import private.key
+          echo "${{ secrets.PRIVATE_KEY_PEM }}" > private.key.pem
+          gpg --import private.key.pem
 
       - name: Run GoReleaser
         id: goreleaser