Build release binaries on Ubuntu 18.04

Author: lnicola

.github/workflows/release.yaml

@@ -30,10 +30,10 @@ jobs:
           - os: windows-latest
             target: aarch64-pc-windows-msvc
             code-target: win32-arm64
-          - os: ubuntu-20.04
+          - os: ubuntu-18.04
             target: x86_64-unknown-linux-gnu
             code-target: linux-x64
-          - os: ubuntu-20.04
+          - os: ubuntu-18.04
             target: aarch64-unknown-linux-gnu
             code-target: linux-arm64
           - os: macos-11