chore: update gitpod.yaml to build multiple packages (aws#25245)

With aws#25224 , gitpod now prebuilds the `aws-cdk-lib` for us and is potentially saving 7-9 minutes with that. This PR makes gitpod continue to build `@aws-cdk-testing/framework-integ` so developers don't have to spend their time waiting for the all integ building. 

Closes #<issue number here>.

----

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

Author: pahud

.gitpod.yml

@@ -6,7 +6,14 @@ github:
 image: jsii/superchain:1-buster-slim-node16
 
 tasks:
-  - init: yarn install && cd packages/aws-cdk-lib && NODE_OPTIONS=--max-old-space-size=8192 ../../scripts/buildup
+  - init: |
+        (
+          set -e
+          yarn install
+          export NODE_OPTIONS=--max-old-space-size=8192
+          # pre-build packages
+          npx lerna run build --scope=aws-cdk-lib --scope=@aws-cdk-testing/framework-integ --skip-nx-cache
+        )
 
 vscode:
   extensions: