(maint) Add linux/macos to github actions for pwsh testing

This commit updates the pwsh module testing to include a matrix of unix, macos
and windows platforms instead of just windows

.github/workflows/pwsh.yaml

@@ -12,7 +12,11 @@ jobs:
 
   test:
     name: Test Generated PowerShell Cmdlets
-    runs-on: windows-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, windows-latest, macos-latest]
+    runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout repository
         uses: actions/checkout@v2