fix(ci): Release using Ubuntu 20.04

Author: epage

.github/workflows/post-release.yml

@@ -61,7 +61,7 @@ jobs:
         build: [linux, macos, win-msvc]
         include:
         - build: linux
-          os: ubuntu-18.04
+          os: ubuntu-20.04
           rust: stable
           target: x86_64-unknown-linux-musl
         - build: macos