move to ubuntu-20.04

Author: sumneko

.github/workflows/build.yml

@@ -20,8 +20,8 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - { os: ubuntu-18.04,   target: linux,   platform: linux-x64    }
-          - { os: ubuntu-18.04,   target: linux,   platform: linux-arm64  }
+          - { os: ubuntu-20.04,   target: linux,   platform: linux-x64    }
+          - { os: ubuntu-20.04,   target: linux,   platform: linux-arm64  }
           - { os: macos-11,       target: darwin,  platform: darwin-x64   }
           - { os: macos-11,       target: darwin,  platform: darwin-arm64 }
           - { os: windows-latest, target: windows, platform: win32-ia32   }