Use container and GKE to accelerate build (#5897)

This reduces the build time from 5 minutes to 3 minutes as we no longer need to cold start a GCE instance. Moreover, we can now compile the host engine using this container.

.cirrus.yml

 gcp_credentials: ENCRYPTED[987a78af29b91ce8489594c9ab3fec21845bbe5ba68294b8f6def3cf0d380830f06687a89ea69c87344c5ade369700fe]
 
-gce_instance:
-  image_project: flutter-cirrus
-  image_name: flutter-engine-ubuntu1604
-  zone: us-central1-a
-  cpu: 24
-  memory: 32Gb
-  disk: 60
+gke_container:
+ image: gcr.io/flutter-cirrus/build-engine-image:latest
+ cluster_name: build-32-cluster
+ zone: us-central1-a
+ namespace: default
+ cpu: 30 # can't use all 30-cores; system pods needs cores too
+ memory: 100Gb # similarly, can't use all 100Gb memory
 
-build_android_task:
+task:
   env:
     CIRRUS_WORKING_DIR: "/tmp/github_repo"
-    ENGINE_PATH: "/var/tmp/flutter/engine"
-    PATH: $PATH:/var/tmp/depot_tools
 
   replace_engine_script: |
     cd $ENGINE_PATH/src
     rm -r flutter
-    cp $CIRRUS_WORKING_DIR -r ./flutter
+    mv $CIRRUS_WORKING_DIR flutter
     gclient sync
 
-  compile_android_script: |
-    cd $ENGINE_PATH/src
-    ./flutter/tools/gn --android --unoptimized
-    ninja -C out/android_debug_unopt
+  matrix:
+    - name: build_host
+      compile_host_script: |
+        cd $ENGINE_PATH/src
+        ./flutter/tools/gn --unoptimized
+        ninja -C out/host_debug_unopt
+    - name: build_android
+      compile_host_script: |
+        cd $ENGINE_PATH/src
+        ./flutter/tools/gn --android --unoptimized
+        ninja -C out/android_debug_unopt