github/workflows: Show context for qemu-arm test failures.

Make it easier to see what went wrong in CI builds.

.github/workflows/ports_qemu-arm.yml

@@ -24,4 +24,4 @@ jobs:
       run: source tools/ci.sh && ci_qemu_arm_build
     - name: Print failures
       if: failure()
-      run: grep --text "FAIL" ports/qemu-arm/build/console.out
+      run: grep --before-context=100 --text "FAIL" ports/qemu-arm/build/console.out