Workflows: Add CI to build Linux binaries

This adds CI to build ROTA from source using Godot Engine, then upload the build as an artifact. Eventually this could be expanded to build a Flatpak, and is a prerequisite to getting it onto Flathub.

I intended to use the [godot-ci GitHub Action](https://github.com/marketplace/actions/godot-ci) for CI, but it doesn't currently support Godot 3.6 (see: abarichello/godot-ci#155). So for now, I'm using [docker.io/smks/godot-ci](https://hub.docker.com/r/smks/godot-ci) which is a container image forked from the former and just updated for 3.6.

.github/workflows/build.yml

@@ -0,0 +1,48 @@
+name: "Build"
+on:
+  workflow_dispatch:
+  pull_request:
+  release:
+    types:
+      - published
+
+# Cancel any ongoing previous run if the job is re-triggered
+concurrency:
+  group: ${{ github.workflow }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+  pages: write
+  id-token: write
+
+env:
+  GODOT_VERSION: 3.6
+
+jobs:
+  linux:
+    name: Linux/X11
+    runs-on: ubuntu-latest
+    container:
+      image: docker.io/smks/godot-ci:3.6
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          lfs: true
+
+      - name: Set up export templates
+        run: |
+          mkdir -v -p ~/.local/share/godot/templates/
+          mv /root/.local/share/godot/templates/${GODOT_VERSION}.stable ~/.local/share/godot/templates/${GODOT_VERSION}.stable
+
+      - name: Build
+        run: |
+          mkdir --verbose --parents ./export/linux
+          godot --no-window --path=./project.godot --export "Linux/X11" ./export/linux/ROTA.x86_64
+
+      - name: Upload artifact
+        uses: actions/upload-pages-artifact@v3
+        with:
+          name: Linux
+          path: export/linux