actions: cron: create /srv/example/cron/contexts first

A loop is waiting for /srv/example/cron-volume/DONE to exist before
declaring the workflow is successfully run. But it is not really until
/srv/example/cron/contexts is created with the event content and they
will race.

Fixes: https://code.forgejo.org/forgejo/end-to-end/issues/181

actions/example-cron/.forgejo/workflows/test.yml

@@ -10,9 +10,6 @@ jobs:
       options: "--volume /srv/example:/srv/example"
 
     steps:
-      - run: |
-          touch /srv/example/cron-volume/DONE
-
       - name: save context
         run: |
           d=/srv/example/cron/contexts/$GITHUB_EVENT_NAME
@@ -20,3 +17,6 @@ jobs:
           cat > $d/github <<'EOF'
           ${{ toJSON(github) }}
           EOF
+
+      - run: |
+          touch /srv/example/cron-volume/DONE