Add GitHub actions

Enable GitHub actions to run CI on all 3 platforms

.github/workflows/build.yml

@@ -0,0 +1,81 @@
+name: Magisk Build
+
+on:
+  push:
+    branches: [ master ]
+  pull_request:
+    branches: [ master ]
+  workflow_dispatch:
+
+jobs:
+  build:
+    name: Build on ${{ matrix.os }}
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ ubuntu-latest, windows-latest, macOS-latest ]
+
+    steps:
+      - name: Check out
+        uses: actions/checkout@v2
+        with:
+          submodules: 'recursive'
+          fetch-depth: 0
+
+      - name: Set up JDK 11
+        uses: actions/setup-java@v1
+        with:
+          java-version: '11'
+
+      - name: Set up Python 3
+        uses: actions/setup-python@v2
+        with:
+          python-version: '3.x'
+
+      - name: Set up GitHub env (Windows)
+        if: runner.os == 'Windows'
+        run: |
+          $oldAndroidPath = $env:ANDROID_SDK_ROOT
+          $sdk_root = "C:\Android"
+          New-Item -Path $sdk_root -ItemType SymbolicLink -Value $oldAndroidPath
+          echo "ANDROID_SDK_ROOT=$sdk_root" >> $env:GITHUB_ENV
+          echo "ANDROID_HOME=$sdk_root" >> $env:GITHUB_ENV
+
+      - name: Set up GitHub env (Unix)
+        if: runner.os != 'Windows'
+        run: echo ANDROID_SDK_ROOT=$ANDROID_SDK_ROOT >> $GITHUB_ENV
+
+      - name: Cache Gradle
+        uses: actions/cache@v2
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+          key: ${{ runner.os }}-gradle-${{ hashFiles('**/*.gradle.kts') }}
+          restore-keys: ${{ runner.os }}-gradle-
+
+      - name: Cache NDK
+        id: ndk-cache
+        uses: actions/cache@v2
+        with:
+          path: ${{ env.ANDROID_SDK_ROOT }}/ndk/magisk
+          key: ${{ runner.os }}-ndk-${{ hashFiles('gradle.properties') }}
+
+      - name: Set up NDK
+        if: steps.ndk-cache.outputs.cache-hit != 'true'
+        run: python3 build.py ndk
+
+      - name: Build release
+        run: python3 build.py -vr all
+
+      - name: Build debug
+        run: python3 build.py -v all
+
+      # Only upload artifacts built on Linux
+      - name: Upload build artifact
+        if: runner.os == 'Linux' && github.event_name != 'pull_request'
+        uses: actions/upload-artifact@v2
+        with:
+          name: ${{ github.sha }}-artifact
+          path: out