Derive pack version from branch name

Note, for pull requests the branch name is the source branch, not the target.
Thus for PRs the pack version will be 0.0.0.

Signed-off-by: Natalie Arellano <narellano@vmware.com>

.github/workflows/build.yml

@@ -49,6 +49,11 @@ jobs:
         run: make verify
       - name: Test
         run: make test
+      - name: Derive pack version from branch name
+        run: |
+          [[ $GITHUB_REF =~ ^refs\/heads\/release/(.*)$ ]] && version=${BASH_REMATCH[1]} || version=0.0.0
+          echo "::set-env name=PACK_VERSION::$version"
+        shell: bash
       - name: Build
         run: make build
         env: