Update QEMU used in CI

I'm getting segfaults on s390x in bytecodealliance#9702 and I can't reproduce them
locally in QEMU. Try updating QEMU to see if it's a transient bug.
Historical updates haven't always gone well, so let's see what CI says.

prtest:full

.github/workflows/main.yml

@@ -659,7 +659,7 @@ jobs:
     name: ${{ matrix.name }}
     runs-on: ${{ matrix.os }}
     env:
-      QEMU_BUILD_VERSION: 8.1.5
+      QEMU_BUILD_VERSION: 9.1.2
     strategy:
       fail-fast: ${{ github.event_name != 'pull_request' }}
       matrix: ${{ fromJson(needs.determine.outputs.test-matrix) }}