Use simpler base image

.github/workflows/test.yml

@@ -132,17 +132,7 @@ jobs:
     steps:
     - name: Checkout the repository
       uses: actions/checkout@master
-    - name: Do Qemu build and test
-      uses: uraimo/run-on-arch-action@v3
-      with:
-        arch: s390x
-        distro: ubuntu_latest
-        dockerRunArgs: |
-          --volume ".:/janet"
-        install: |
-          apt-get -y update
-          apt-get -y install git build-essential
-        run: |
-          cd /janet
-          make -j3
-          make test
+    - name: Enable qemu
+      run: docker run --privileged --rm tonistiigi/binfmt --install s390x
+    - name: Build and run on emulated architecture
+      run: docker run --rm -v .:/janet --platform linux/s390x ubuntu bash -c "apt-get -y update && apt-get -y install git build-essential && cd /janet && make -j3 && make test"