Add CI that verifies the repository builds

This runs on Arch, and shows that things build on another distro with
recent dependencies. If we want to support older versions of
dependencies, that will need to be tested on a different platform.

This also shows what dependencies are needed.

.github/workflows/ci.yml

@@ -0,0 +1,46 @@
+# Use Arch to test build outside of Ubuntu/Pop.
+#
+# Tests with recent version of dependencies. Another testson a different
+# distro could be used to test for lowest supported.
+#
+# A minimal image should also help to confirm what dependencies are needed.
+
+name: CI
+
+on:
+  pull_request:
+  push:
+  schedule:
+    - cron: '0 7 * * *' # Midnight MST daily
+
+jobs:
+  Test:
+    runs-on: ubuntu-latest
+    container:
+      image: archlinux:latest
+    steps:
+    - run: >
+        pacman --noconfirm -Syu
+        base-devel
+        cargo
+        clang
+        desktop-file-utils
+        git
+        gtk4
+        just
+        libinput
+        libxkbcommon
+        llvm
+        mesa
+        meson
+        pipewire
+        pulseaudio
+        seatd
+        wayland
+    - uses: actions/checkout@v3
+      with:
+        submodules: true
+    # Safe directory behavior seems to have issues with `container:`
+    # https://github.com/actions/checkout/issues/915
+    - run: git config --global --add safe.directory '*'
+    - run: just sysext