Integrity check to generate zoom 14 tiles (#1231)

Setting the MIN/MAX zoom levels in the `env` section had no effect since the values in the `.env` file have higher precedence.

See the currently effective `maxzoom = 7` value in [a typical integrity check log]( https://github.com/openmaptiles/openmaptiles/pull/1223/checks#step:3:3546).

.github/workflows/integrity.yml

@@ -20,10 +20,10 @@ jobs:
       - name: Run quickstart for a small area
         env:
           area: monaco
-          MIN_ZOOM: 0
-          MAX_ZOOM: 14
           QUIET: 1
         run: |
+          echo MIN_ZOOM=0 >> .env
+          echo MAX_ZOOM=14 >> .env
           ./quickstart.sh $area
 
       - name: Save quickstart.log