Actually run GH workflows on different OS

Change-Id: Ib96a13ea388ec4e8dc46e5baddb874fe5aea161e
diff --git a/.github/workflows/ci_test.yml b/.github/workflows/ci_test.yml
index 78e1bf8..6c82e1c 100644
--- a/.github/workflows/ci_test.yml
+++ b/.github/workflows/ci_test.yml
@@ -8,7 +8,7 @@
   pull_request:
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
     strategy:
       matrix:
         os: ['ubuntu-latest', 'macos-latest', 'windows-latest']