run ci on darwin and add dependabot

.github/dependabot.yml

@@ -0,0 +1,6 @@
+version: 2
+updates:
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"

.github/workflows/ci.yml

@@ -1,11 +1,16 @@
 name: "Test"
 on:
-  pull_request:
+  pull_request_target:
   push:
+    branches:
+      - 'master'
 jobs:
   tests:
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [ubuntu-latest, macos-latest]
     steps:
-    - uses: actions/checkout@v2
-    - uses: cachix/install-nix-action@v16
-    - run: nix build -L -f . -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz --show-trace --experimental-features 'nix-command flakes'
+    - uses: actions/checkout@v3
+    - uses: cachix/install-nix-action@v17
+    - run: nix build --extra-experimental-features nix-command -L -f . -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz --show-trace