CI: Add Linux headless/server builds

Adapted from linux_builds.yml.

.github/workflows/server_builds.yml

@@ -0,0 +1,117 @@
+name: ☁ Server 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: Linux Headless 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-headless-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=server tools=yes target=release_debug module_mono_enabled=yes mono_glue=no
+
+  linux-server:
+    runs-on: "ubuntu-20.04"
+    name: Linux Server 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-server-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=server target=release tools=no module_mono_enabled=yes mono_glue=no