Try running GH Action on Windows in addition to Linux.

.github/workflows/ci.yml

@@ -10,7 +10,10 @@ on:
 
 jobs:
   main:
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os: [ubuntu-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
       - name: Use Node.js 15
@@ -25,7 +28,7 @@ jobs:
           path: |
             ~/.cache/Cypress
             node_modules
-          key: node_modules-node-v${{ matrix.node }}-${{ hashFiles('package-lock.json') }}
+          key: node_modules-${{ matrix.os }}-node-v${{ matrix.node }}-${{ hashFiles('package-lock.json') }}
 
       # Re-use ~/.elm between runs until elm.json, elm-tooling.json or
       # review/elm.json changes. The Elm compiler saves downloaded Elm packages
@@ -34,7 +37,7 @@ jobs:
         uses: actions/cache@v2
         with:
           path: ~/.elm
-          key: elm-${{ hashFiles('elm.json', 'elm-tooling.json', 'generator/elm.json', 'review/elm.json') }}
+          key: elm-${{ matrix.os }}-${{ hashFiles('elm.json', 'elm-tooling.json', 'generator/elm.json', 'review/elm.json') }}
 
       # Install npm packages, unless we restored them from cache.
       # Since `npm ci` removes the node_modules folder before running it’s