ci: initial setup (#21)

.github/dependabot.yml

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

.github/workflows/nix.yml

@@ -0,0 +1,23 @@
+name: Nix
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+  workflow_dispatch:
+jobs:
+  build:
+    strategy:
+      matrix:
+        os: [ ubuntu-20.04 ]
+    runs-on: ${{ matrix.os }}
+    steps:
+      - uses: actions/checkout@v2
+      - uses: cachix/install-nix-action@v12
+      - uses: cachix/cachix-action@v8
+        with:
+          name: numtide
+          signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+      - run: nix-shell --pure --run "cargo build"
+      - run: nix-shell --pure --run "cargo test"
+      - run: nix-build -A defaultNix.packages.x86_64-linux