Run CI on all operating systems

Closes #309

Author: hinerm

.github/workflows/build-main.yml

@@ -8,8 +8,11 @@ on:
       - "*-[0-9]+.*"
 
 jobs:
-  build:
-    runs-on: ubuntu-latest
+  define-matrix:
+    runs-on: ${{ matrix.os }}
+    strategy:
+        matrix:
+          os: [ubuntu-latest, windows-latest, macOS-latest]
 
     steps:
       - uses: actions/checkout@v2

.github/workflows/build-pr.yml

@@ -6,8 +6,11 @@ on:
       - master
 
 jobs:
-  build:
-    runs-on: ubuntu-latest
+  define-matrix:
+    runs-on: ${{ matrix.os }}
+    strategy:
+        matrix:
+          os: [ubuntu-latest, windows-latest, macOS-latest]
 
     steps:
       - uses: actions/checkout@v2