deploy: prune app engine versions during build

The daily playground-deploy-gotip-playground build job caused
the golang-org project to reach the per project app engine
version limit of 210. This change adds a step to prune versions
using the command from x/website.

Change-Id: I2384c7af97ab1b8f4e5ebad8ee10e41227a506a6
Reviewed-on: https://go-review.googlesource.com/c/playground/+/398574
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: jamalc

deploy/deploy_gotip.json

@@ -24,6 +24,14 @@
         "--project=$PROJECT_ID",
         "--image-url=gcr.io/$PROJECT_ID/playground-gotip:latest"
       ]
+    },
+    {
+      "name": "golang",
+      "entrypoint": "sh",
+      "args": [
+        "-c",
+        "go install golang.org/x/website/cmd/versionprune@latest && versionprune --dry_run=false --project=$PROJECT_ID --service=gotipplay"
+      ]
     }
   ],
   "timeout": "3600s",