Use ubuntu:18.04.

Author: piobab

.github/workflows/release.yml

@@ -12,10 +12,10 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-18.04
             target: x86_64-unknown-linux-gnu
 
-          - os: ubuntu-latest
+          - os: ubuntu-18.04
             target: x86_64-unknown-linux-musl
 
           - os: macos-latest