Update build_image.yml

Author: jacobbednarz

.github/workflows/build_image.yml

@@ -44,7 +44,7 @@ jobs:
           outputs: type=image,name=${{ env.REGISTRY_IMAGE }},push-by-digest=true,name-canonical=true,push=true
       - name: Set artifact identifier
         run: |
-          platform=$(${{ matrix.platform }} | tr / -)
+          platform=$(echo ${{ matrix.platform }} | tr / -)
           echo "artifact_identifier=$platform" >> "$GITHUB_ENV"
       - name: Export digest
         run: |