chore: enable gitpod prebuilds for PRs from forks (aws#10824)

This enables GitPod pre-builds to be created for PRs that are created
from forks.

Docs: https://www.gitpod.io/docs/prebuilds/#configure-the-github-app


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

.gitpod.yml

@@ -1,4 +1,10 @@
+github:
+  prebuilds:
+    pullRequestsFromForks: true
+    addComment: true
+
 image: jsii/superchain
+
 tasks:
   - init: yarn build --skip-test --no-bail --skip-prereqs --skip-compat