Make it build on Windows too

.github/workflows/gradle.yml

@@ -2,9 +2,22 @@ name: Gradle Build
 on: [push, pull_request]
 
 jobs:
-  build:
+  build-ubuntu:
+    name: Build WorldEdit on Ubuntu (Latest)
     runs-on: ubuntu-latest
 
+    steps:
+    - uses: actions/checkout@v2
+    - name: Set up JDK 1.8
+      uses: actions/setup-java@v1
+      with:
+        java-version: 1.8
+    - name: Build with Gradle
+      run: ./gradlew build
+  build-windows:
+    name: Build WorldEdit on Windows (Latest)
+    runs-on: windows-latest
+
     steps:
     - uses: actions/checkout@v2
     - name: Set up JDK 1.8