switch to github actions from travis

.github/workflows/pr.yml

@@ -0,0 +1,12 @@
+name: "Test"
+on:
+  pull_request:
+jobs:
+  tests:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - uses: cachix/install-nix-action@v12
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - run: ./ci/test.sh