CI: Specify Ubuntu version

We don't want to deal with version issues.
We have ancient tools :-(.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>

Author: FlyGoat

.github/workflows/compile.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
 
     steps:
     - uses: actions/checkout@v2