Configured outputs for Admin build step

ref https://linear.app/tryghost/issue/DEV-20/faster-builds

- by doing this, we can tell Nx which files should be restored when the
  cache is restored

ghost/admin/package.json

@@ -193,6 +193,10 @@
         ]
       },
       "build": {
+        "outputs": [
+          "{projectRoot}/dist",
+          "{workspaceRoot}/ghost/core/core/built/admin"
+        ],
         "dependsOn": [
           "build",
           {