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/linux_builds.yml

@@ -0,0 +1,117 @@
+name: Linux Builds
+on: [push, pull_request]
+
+# Global Cache Settings
+env:
+  GODOT_BASE_BRANCH: 3.2
+  SCONS_CACHE_LIMIT: 4096
+
+jobs:
+  linux-editor:
+    runs-on: "ubuntu-20.04"
+    name: Editor w/ Mono (target=release_debug, tools=yes)
+
+    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 build-essential pkg-config libx11-dev libxcursor-dev \
+            libxinerama-dev libgl1-mesa-dev libglu-dev libasound2-dev libpulse-dev libudev-dev libxi-dev libxrandr-dev yasm
+
+      # Upload cache on completion and check it out now
+      - name: Load .scons_cache directory
+        id: linux-editor-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; best to keep self contained in it's own step)
+      - 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'
+
+      # Setup scons, print python version and scons version info, so if anything is broken it won't run the build.
+      - name: Configuring Python packages
+        run: |
+          python -c "import sys; print(sys.version)"
+          python -m pip install scons
+          python --version
+          scons --version
+
+      # We should always be explicit with our flags usage here since it's gonna be sure to always set those flags
+      - name: Compilation
+        env:
+          SCONS_CACHE: ${{github.workspace}}/.scons_cache/
+        run: |
+          scons -j2 verbose=yes warnings=all werror=yes platform=x11 tools=yes target=release_debug module_mono_enabled=yes mono_glue=no
+
+  linux-template:
+    runs-on: "ubuntu-20.04"
+    name: Template w/ Mono (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 build-essential pkg-config libx11-dev libxcursor-dev \
+            libxinerama-dev libgl1-mesa-dev libglu-dev libasound2-dev libpulse-dev libudev-dev libxi-dev libxrandr-dev yasm
+
+      # Upload cache on completion and check it out now
+      - name: Load .scons_cache directory
+        id: linux-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=x11 target=release tools=no module_mono_enabled=yes mono_glue=no