Add rust-src toolchain component

Provides sources of standard library, e.g. for rust-analyzer.

rust/flake.nix

@@ -22,13 +22,17 @@
           rustToolchain =
             let
               rust = super.rust-bin;
-            in
+              toolchain =
                 if builtins.pathExists ./rust-toolchain.toml then
                   rust.fromRustupToolchainFile ./rust-toolchain.toml
                 else if builtins.pathExists ./rust-toolchain then
                   rust.fromRustupToolchainFile ./rust-toolchain
                 else
                   rust.stable.latest.default;
+            in
+            toolchain.override {
+              extensions = [ "rust-src" ]; # https://github.com/the-nix-way/dev-templates/issues/4
+            };
         })
       ];