feat: Add lake CI

.github/workflows/build.yml

@@ -0,0 +1,37 @@
+on:
+  push:
+    branches-ignore:
+      # ignore tmp branches used by bors
+      - 'staging.tmp*'
+      - 'trying.tmp*'
+      - 'staging*.tmp'
+  pull_request:
+
+name: ci
+
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    env:
+      NIX_BUILD_ARGS: -v --print-build-logs --fallback --show-trace
+    steps:
+      - uses: cachix/install-nix-action@v16
+        with:
+          install_url: https://nixos-nix-install-tests.cachix.org/serve/i6laym9jw3wg9mw6ncyrk6gjx4l34vvx/install
+          install_options: '--tarball-url-prefix https://nixos-nix-install-tests.cachix.org/serve'
+          extra_nix_config: |
+            experimental-features = nix-command flakes ca-references
+      - name: install elan
+        run: |
+          set -o pipefail
+          curl -sSfL https://github.com/leanprover/elan/releases/download/v1.4.2/elan-x86_64-unknown-linux-gnu.tar.gz | tar xz
+          ./elan-init -y --default-toolchain none
+          echo "$HOME/.elan/bin" >> $GITHUB_PATH
+
+      - uses: actions/checkout@v2
+      - name: build
+        run: nix shell $NIX_BUILD_ARGS --command lake build
+
+      - name: test SDL
+        run: nix shell $NIX_BUILD_ARGS --command lake exe Tests