Fix for container rc tag (Attempt 2) (#398)

## Summary

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This is the same fix as #389 but applied to the RC workflow rather than
the release workflow as was the original intent with #389. Both
workflows need this change so not reverting the other one.

---

- [x] "I certify that all code in this PR is my own, except as noted
below."

## Use of AI

- [ ] Includes AI-assisted code completion
- [ ] Includes code generated by an AI application
- [ ] Includes AI-generated tests (NOTE: AI written tests should have a
docstring that includes `## WRITTEN BY AI ##`)

Signed-off-by: Samuel Monson <[email protected]>

Author: sjmonson

.github/workflows/release-candidate.yml

@@ -298,7 +298,10 @@ jobs:
         with:
           fetch-depth: 0
       - name: Get version from branch
-        run: echo "PACKAGE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
+        run: |
+          GITHUB_REF="${{ github.ref }}"
+          [[ -z "$GITHUB_REF" ]] && exit 1 # Fail if ref is unset
+          echo "PACKAGE_VERSION=${GITHUB_REF#refs/*/}" >> $GITHUB_ENV
       - name: Buildah build
         id: build-image
         uses: redhat-actions/buildah-build@v2