Add Ubuntu 18.04

Just in case, we'd like to make sure the SDK is buildable on Ubuntu 18.04.

Signed-off-by: Kazuyoshi Kato <[email protected]>

Author: kzys

.github/workflows/go.yml

@@ -12,7 +12,7 @@ jobs:
     strategy:
       matrix:
         go: ['1.13', '1.14', '1.15', '1.16', '1.17']
-        os: ['ubuntu-20.04']
+        os: ['ubuntu-18.04', 'ubuntu-20.04']
       fail-fast: false
     name: ${{ matrix.os }} / Go ${{ matrix.go }}
     runs-on: ${{ matrix.os }}