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 +1,55 @@
+# Keep sorted
+{ cargo-deb
+, default
+, engage
+, go
+, inputs
+, jq
+, lychee
+, mdbook
+, mkShell
+, olm
+, 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 = "${toolchain}/lib/rustlib/src/rust/library";
+  };
+
+  # Development tools
+  nativeBuildInputs = default.nativeBuildInputs ++ [
+    # Always use nightly rustfmt because most of its options are unstable
+    #
+    # This needs to come before `toolchain` in this list, otherwise
+    # `$PATH` will have stable rustfmt instead.
+    inputs.fenix.packages.${system}.latest.rustfmt
+
+    # Keep sorted
+    engage
+    toolchain
+
+    # Needed for producing Debian packages
+    cargo-deb
+
+    # Needed for our script for Complement
+    jq
+
+    # Needed for Complement
+    go
+    olm
+
+    # Needed for our script for Complement
+    jq
+
+    # Needed for finding broken markdown links
+    lychee
+
+    # Useful for editing the book locally
+    mdbook
+  ];
+}