ci: build code again on main to pre-fill the rosetta cache (#34834)

### Reason for this change

[Restrictions for accessing a cache](https://docs.github.com/en/actions/how-tos/writing-workflows/choosing-what-your-workflow-does/caching-dependencies-to-speed-up-workflows#restrictions-for-accessing-a-cache) effectively limit a PR to use the rosetta cache either from the same branch or the main branch. Because we currently don't fill the rosetta cache on main, most PRs cannot use this cache.

Using the cache removes ~14min of most builds. This is a worthwhile investment.

### Description of changes

Run the PR build against `main` and `v2-release` to always fill up the rosetta cache. We could probably set this up to skip tests, but for now this is easier.

Also adds manual (always helpful) and merge group trigger (future proofing) to the build.

### Describe any new or updated permissions being added

n/a

### Description of how you validated changes

Reading the GitHub docs. Needs merging to actually validate.

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

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

Author: mrgrain

.github/workflows/pr-build.yml

@@ -1,6 +1,12 @@
 name: PR Build
 
 on:
+  workflow_dispatch: {}
+  merge_group: {}
+  push:
+    branches:
+      - main
+      - v2-release
   pull_request:
     branches:
       - main