Add CI workflow

.tangled/workflows/build.yaml

···
+when:
+  - event: ["push", "pull_request"]
+    branch: ["main"]
+  - event: ["manual"]
+
+dependencies:
+  nixpkgs:
+    - rustc
+    - cargo
+
+steps:
+  - name: check build
+    command: |
+      cargo check