Move CI to Github Actions

This deletes the travis yaml file & introduces a simple GHA workflow
which runs the build in the Nix devShell.

Author: matthew-healy

.github/actions/nix-setup/action.yaml

@@ -0,0 +1,10 @@
+name: Set up Nix
+description: Sets up Nix and Nix store caching
+
+runs:
+  using: "composite"
+  steps:
+    - uses: DeterminateSystems/nix-installer-action@main
+    - uses: DeterminateSystems/magic-nix-cache-action@main
+      with:
+        use-flakehub: false

.github/workflows/ci.yaml

@@ -0,0 +1,17 @@
+name: CI
+
+on:
+  pull_request:
+  push:
+    branches: [main]
+
+jobs:
+  ci:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ./.github/actions/nix-setup
+      - name: Run tests
+        run: |
+          nix develop -c \
+            bin/build