chore: disable codecov workflow on forks (#32109)

We shouldn't be trying to publish codecov results when the workflow runs in a fork. Nevermind that it will most likely fail because a token is required. 

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Author: iliapolo

.github/workflows/codecov.yml

@@ -9,6 +9,7 @@ on:
 jobs:
   collect:
     name: collect
+    if: github.repository == 'aws/aws-cdk'
     runs-on: ubuntu-latest
     steps:
       - name: Checkout