chore(build): Update cloudbuild.yaml file (#7282)

The currently checked-in Google Cloud Build configs have not been maintained and
are no longer used when producing the official Spinnaker releases. Instead, the
build process generates a GCB config on-the-fly and uses that.

To make the process more clear and make it easier for others to reproduce the
builds, commit the GCB config that is auto-generated to the repo. Also, delete
the unused cloudbuild-tagged.yaml.

cloudbuild.yaml

@@ -1,11 +1,13 @@
 steps:
-- name: 'openjdk:8-stretch'
-  env: ["GRADLE_USER_HOME=cache"]
-  entrypoint: "bash"
-  args: [ "-c", "./gradlew build -PskipTests"]
-- name: 'gcr.io/cloud-builders/docker'
-  args: ["build", "-t", "gcr.io/$PROJECT_ID/$REPO_NAME:$COMMIT_SHA", "-t", "gcr.io/$PROJECT_ID/$REPO_NAME:latest", "-f", "Dockerfile.slim", "."]
+  - name: gcr.io/spinnaker-marketplace/gradle_cache
+    env: ['GRADLE_USER_HOME=/gradle_cache/.gradle']
+    entrypoint: 'bash'
+    args: ['-c', './gradlew build -PskipTests']
+  - name: gcr.io/cloud-builders/docker
+    args: ['build', '-t', 'gcr.io/$PROJECT_ID/$REPO_NAME:$TAG_NAME', '-f', 'Dockerfile.slim', '.']
+    env: ['GRADLE_USER_HOME=/gradle_cache/.gradle']
 images:
-- 'gcr.io/$PROJECT_ID/$REPO_NAME:$COMMIT_SHA'
-- 'gcr.io/$PROJECT_ID/$REPO_NAME:latest'
-timeout: 15m
+  - gcr.io/$PROJECT_ID/$REPO_NAME:$TAG_NAME
+timeout: 3600s
+options:
+  machineType: N1_HIGHCPU_8