Add CI to verify Nix build.

.github/workflows/build.yaml

@@ -0,0 +1,16 @@
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  lints:
+    name: Build
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+      - uses: DeterminateSystems/nix-installer-action@main
+      # Verify that we can build the escript from the flake.
+      # This is primarily a test of the Mix dependencies hash being up-to-date
+      # in the flake.
+      - run: nix build .