fix(ci): Release using Ubuntu 20.04

Author: epage

.github/workflows/post-release.yml

@@ -79,7 +79,7 @@ jobs:
       with:
         fetch-depth: 1
     - name: Install packages (Ubuntu)
-      if: matrix.os == 'ubuntu-18.04'
+      if: matrix.os == 'ubuntu-20.04'
       run: |
         sudo apt-get update
         sudo apt-get install -y --no-install-recommends xz-utils liblz4-tool musl-tools