add CI

.github/workflows/build.yml

@@ -0,0 +1,22 @@
+on:
+  push:
+  pull_request:
+
+name: ci
+
+jobs:
+  build:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - 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 package
+        run: cd Lean && lake exe cache get && lake build