ci: Run tests

We were not running tests in ci, so fix this, also add a step just to
enter the nix dev environment so the we know how long does it take to
pull / build our native dependencies and tooling.

Also changing `check` -> `build` as the CLI integration tests require
the debug binary to be built. This is a bit janky and we should have
better integration with the build system. We'll change this later on.

Test Plan
=========

ci :)

.github/workflows/build.yml

@@ -13,10 +13,14 @@ jobs:
     - uses: actions/checkout@main
     - uses: DeterminateSystems/nix-installer-action@main
     - uses: DeterminateSystems/magic-nix-cache-action@main
-    - name: Run `cargo check`
-      run: nix develop --command cargo check
+    - name: Set up nix dev env
+      run: nix develop --command echo 0
+    - name: Run `cargo build`
+      run: nix develop --command cargo build
     - name: Run `cargo clippy`
       run: nix develop --command cargo clippy --all-targets -- -D warnings
+    - name: Run `cargo test`
+      run: nix develop --command cargo test
     - name: Run `cargo fmt`
       run: nix develop --command cargo fmt --check
     - name: Run `nix fmt`