gha/.build: derive GIT_TAG from PKG_REF

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

Author: vvoland

.github/workflows/.build.yml

@@ -216,9 +216,9 @@ jobs:
             echo "${l?}" >> $GITHUB_ENV
           done
           if [ "${{ github.event_name }}" = "schedule" ]; then
-            echo "GIT_TAG=nightly/${{ inputs.name }}/$VERSION" >> $GITHUB_ENV
+            echo "GIT_TAG=nightly/${{ inputs.name }}/$PKG_REF" >> $GITHUB_ENV
           else
-            echo "GIT_TAG=${{ inputs.name }}/$VERSION" >> $GITHUB_ENV
+            echo "GIT_TAG=${{ inputs.name }}/$PKG_REF" >> $GITHUB_ENV
           fi
       -
         name: Docker meta