ci: Fix syntax (#15709)

Signed-off-by: Jack Baldry <jack.baldry@grafana.com>

.github/workflows/deploy-pr-preview.yml

@@ -11,7 +11,7 @@ on:
 
 jobs:
   deploy-pr-preview:
-    if: ! github.event.pull_request.head.repo.fork
+    if: ${{ ! github.event.pull_request.head.repo.fork }}
     uses: grafana/writers-toolkit/.github/workflows/deploy-preview.yml@main
     with:
       sha: ${{ github.event.pull_request.head.sha }}