(CI)(modified-packages) fix missing BRANCH variable

Author: MatthijsBurgh

ci/modified-packages/action.yml

@@ -19,8 +19,9 @@ runs:
       run: |
         PULLREQUEST=${{ github.event.number }}
         PULLREQUEST=${PULLREQUEST:-false}
+        BRANCH=${GITHUB_BASE_REF:-${GITHUB_REF}}
         DEFAULT_BRANCH=${{ github.event.repository.default_branch }}
-        if [[ ${{ github.event_name }} == "schedule" ]] || ([[ ${{ github.event_name }} == "push" ]] && [[ "$BRANCH" == "$DEFAULT_BRANCH" ]])
+        if [[ "${GITHUB_EVENT_NAME}" == "schedule" ]] || ([[ "${GITHUB_EVENT_NAME}" == "push" ]] && [[ "${BRANCH}" == "${DEFAULT_BRANCH}" ]])
         then
             ALL=true
         fi