Fix maintainer check condition (#32110)

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## Summary

I've noticed that the value stored under `is_core_team` gets
stringified, so some PRs may be mislabelled as coming from the core
team.

I've checked this on my fork and saw stringified `null` returned by the
`is_core_team`, and this PR explicitly checks for the correct value.
Feel free to close this PR if you want to go with another approach.

## How did you test this change?

Checked this change on my fork with and without listing myself in the
maintainers file.

.github/workflows/shared_label_core_team_prs.yml

@@ -13,7 +13,7 @@ jobs:
     uses: facebook/react/.github/workflows/shared_check_maintainer.yml@main
 
   label:
-    if: ${{ needs.check_maintainer.outputs.is_core_team }}
+    if: ${{ needs.check_maintainer.outputs.is_core_team == 'true' }}
     runs-on: ubuntu-latest
     needs: check_maintainer
     steps: