ci: limit linux/arm64 builds to pushes to main to reduce build times on PRs

Author: tangowithfoxtrot

.github/workflows/build-cli-docker.yml

@@ -85,15 +85,24 @@ jobs:
             echo "tags=$_AZ_REGISTRY/bws:${IMAGE_TAG}" >> $GITHUB_OUTPUT
           fi
 
+      # it takes significantly longer to build for the linux/arm64 platform,
+      # so we only build for both platforms on pushes to main
+      - name: Set build targets based on workflow trigger
+        id: platforms
+        run: |
+          if [[ "$GITHUB_EVENT_NAME" == "pull_request" ]]; then
+            echo "platforms=linux/amd64" >> $GITHUB_OUTPUT
+          else
+            echo "platforms=linux/amd64,linux/arm64/v8" >> $GITHUB_OUTPUT
+          fi
+
       - name: Build and push Docker image
         id: build-docker
         uses: docker/build-push-action@4f58ea79222b3b9dc2c8bbdd6debcef730109a75 # v6.9.0
         with:
           context: .
           file: crates/bws/Dockerfile
-          platforms: |
-            linux/amd64,
-            linux/arm64/v8
+          platforms: ${{ steps.platforms.outputs.platforms }}
           push: true
           tags: ${{ steps.tag-list.outputs.tags }}
           secrets: |