Update pr-merge.yaml - shell bash

.github/workflows/pr-merge.yaml

@@ -8,6 +8,7 @@
         runs-on: ubuntu-latest
         steps:
         - name: Send Discord notification
+          shell: bash
           env:
             DISCORD_WEBHOOK_BETA_TESTERS: ${{ secrets.DISCORD_WEBHOOK_BETA_TESTERS }}
           if: github.event.pull_request.merged == true