use ubuntu-18.04 for host machine, in order to use previous qemu version https://launchpad.net/qemu/+packages

Author: k-okada

.github/workflows/config.yml

@@ -64,7 +64,7 @@ jobs:
           # sparc64
       fail-fast: false
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     timeout-minutes: 60
 
     name: linux