ci: prevent staging from recreating the image

The develop workflow creates the image, so there's no need for the staging workflow to recreate it.

If the image hasn't been created by the develop workflow, then the deployment will fail. This should be the expected behaviour, rather than creating the image and not testing it fully.

Refs #395

.github/workflows/deploy_prereview-staging.yml

@@ -1,47 +1,18 @@
-name: Build and deploy to staging
+name: Deploy to staging
 
 on:
   push:
     branches:
       - main
 
 env:
-  DOCKER_BUILDKIT: 1
   IMAGE_TAG: ${{ github.sha }}
   REGISTRY_USERNAME: prereview
 
 jobs:
 
-  build:
-    runs-on: ubuntu-20.04
-
-    steps:
-      - name: 'Checkout code'
-        uses: actions/checkout@v2
-        with:
-          lfs: true
-
-      - name: 'Build image'
-        run: make build
-        env:
-          TARGET: 'prod'
-
-      - name: 'Smoke test'
-        run: .github/smoke-test.sh
-
-      - name: 'Log in to registry'
-        uses: azure/docker-login@v1
-        with:
-          login-server: prereview.azurecr.io
-          username: ${{ env.REGISTRY_USERNAME }}
-          password: ${{ secrets.REGISTRY_PASSWORD }}
-
-      - name: 'Push image'
-        run: docker push prereview.azurecr.io/prereview:${{ env.IMAGE_TAG }}
-
   deploy:
     runs-on: ubuntu-20.04
-    needs: build
 
     steps:
       - name: 'Checkout code'