fix test matrix (#4)

* fix runs on

* Update go.yml

Author: Fs02

.github/workflows/go.yml

@@ -14,7 +14,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
         go: [1.12, 1.13, 1.4, 1.15]
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     steps:
     - name: Set up Go 1.x
       uses: actions/setup-go@v2