Pin first-contribution action

Workaround for plbstl/first-contribution#106

Author: melissawm

.github/workflows/pr_welcome.yml

@@ -16,7 +16,7 @@ jobs:
       issues: write
       pull-requests: write
     steps:
-      - uses: plbstl/first-contribution@7c31f41b0e7a70adfcae06cf964679f61af6780b  # v4.3.0
+      - uses: plbstl/first-contribution@4fb1541ce2706255850d56c5684552607be1ae9b  # v4.2.0
         with:
           labels: first-contribution
           pr-opened-msg: >+