Set default shell to bash

This sets the `-o pipefail` behavior that you might expect to be a default (see https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#exit-codes-and-error-action-preference). Without this setting, `go test -count=1 -v -tags corpus ./integration_tests/... | tail` will always return a success error code.

.github/workflows/corpus.yml

@@ -7,7 +7,11 @@ on:
 
 jobs:
   build:
+    defaults:
+      run:
+        shell: bash
     runs-on: ubuntu-latest
+
     steps:
       - uses: actions/checkout@v4