CI: Backport GitHub Actions setup from master

Removes AppVeyor and all Travis jobs but the iOS one, which hasn't been ported
to GitHub Actions yet (should be done soon).

Backports new style scripts from `master` branch too to do the same checks.

.github/workflows/android_builds.yml

@@ -0,0 +1,72 @@
+name: Android Builds
+on: [push, pull_request]
+
+# Global Cache Settings
+env:
+  GODOT_BASE_BRANCH: 3.2
+  SCONS_CACHE_LIMIT: 4096
+
+jobs:
+  android-template:
+    runs-on: "ubuntu-20.04"
+
+    name: Template (target=release, tools=no)
+
+    steps:
+      - uses: actions/checkout@v2
+
+      # Azure repositories are not reliable, we need to prevent azure giving us packages.
+      - name: Make apt sources.list use the default Ubuntu repositories
+        run: |
+          sudo cp -f misc/ci/sources.list /etc/apt/sources.list
+          sudo apt-get update
+
+      # Install all packages (except scons)
+      - name: Configure dependencies
+        run: |
+         sudo apt-get install openjdk-8-jdk
+         echo "::set-env name=JAVA_HOME::usr/lib/jvm/java-8-openjdk-amd64"
+
+      - name: Install Android SDK and NDK
+        run: |
+          echo "::set-env name=PATH::/usr/lib/jvm/java-8-openjdk-amd64/jre/bin:${PATH}"
+          java -version
+          echo "::set-env name=ANDROID_HOME::$(pwd)/godot-dev/build-tools/android-sdk"
+          echo "::set-env name=ANDROID_NDK_ROOT::$(pwd)/godot-dev/build-tools/android-ndk"
+          misc/ci/android-tools-linux.sh
+          source ~/.bashrc
+
+      # Upload cache on completion and check it out now
+      - name: Load .scons_cache directory
+        id: android-template-cache
+        uses: actions/cache@v2
+        with:
+          path: ${{github.workspace}}/.scons_cache/
+          key: ${{github.job}}-${{env.GODOT_BASE_BRANCH}}-${{github.ref}}-${{github.sha}}
+          restore-keys: |
+            ${{github.job}}-${{env.GODOT_BASE_BRANCH}}-${{github.ref}}-${{github.sha}}
+            ${{github.job}}-${{env.GODOT_BASE_BRANCH}}-${{github.ref}}
+            ${{github.job}}-${{env.GODOT_BASE_BRANCH}}
+
+      # Use python 3.x release (works cross platform)
+      - name: Set up Python 3.x
+        uses: actions/setup-python@v2
+        with:
+          # Semantic version range syntax or exact version of a Python version
+          python-version: '3.x'
+          # Optional - x64 or x86 architecture, defaults to x64
+          architecture: 'x64'
+
+      # You can test your matrix by printing the current Python version
+      - name: Configuring Python packages
+        run: |
+          python -c "import sys; print(sys.version)"
+          python -m pip install scons
+          python --version
+          scons --version
+
+      - name: Compilation
+        env:
+          SCONS_CACHE: ${{github.workspace}}/.scons_cache/
+        run: |
+          scons -j2 verbose=yes warnings=all werror=yes platform=android target=release tools=no