factor out nix code into new files via makeScope

This makes the Nix code a lot easier to reason about.

nix/shell.nix

@@ -0,0 +1,34 @@
+# Keep sorted
+{ default
+, engage
+, inputs
+, jq
+, mkShell
+, system
+, toolchain
+}:
+
+mkShell {
+  env = default.env // {
+    # Rust Analyzer needs to be able to find the path to default crate
+    # sources, and it can read this environment variable to do so. The
+    # `rust-src` component is required in order for this to work.
+    RUST_SRC_PATH = "${toolchain}/lib/rustlib/src/rust/library";
+  };
+
+  # Development tools
+  nativeBuildInputs = [
+    # Always use nightly rustfmt because most of its options are unstable
+    #
+    # This needs to come before `toolchain` in this list, otherwise
+    # `$PATH` will have stable rustfmt instead.
+    inputs.fenix.packages.${system}.latest.rustfmt
+
+    # Keep sorted
+    engage
+    jq
+    toolchain
+  ]
+  ++
+  default.nativeBuildInputs;
+}