fix env var in action

Author: joemaller

.github/workflows/push-to-dockerhub.yml

@@ -54,8 +54,8 @@ jobs:
             type=semver,pattern={{version}}
             type=semver,pattern={{major}}.{{minor}}
             type=semver,pattern={{major}}
-            type=raw,${{ WP_VERSION }}-{{version}}
-            type=raw,${{ WP_VERSION }}
+            type=raw,${{ env.WP_VERSION }}-{{version}}
+            type=raw,${{ env.WP_VERSION }}
 
       - name: Login to Docker Hub
         uses: docker/login-action@v1