Directly use the toolchain as given

Previously we hardcoded the nightly chain. This might not be true in the
future!

Signed-off-by: Marcel Müller <neikos@neikos.email>

lib/mk-cargo-dylint-driver.nix

@@ -1,13 +1,24 @@
 {
+  pkgs,
   craneLib,
 }:
 
-toolchainName: toolchain:
+toolchainName:
 
 let
   pname = "cargo-dylint-driver-${toolchainName}";
   version = "4.0.0";
   src = ../dylint_driver;
+  toolchain =
+    (pkgs.rust-bin.fromRustupToolchainFile (
+      pkgs.writeText "${toolchainName}-toolchain.toml" toolchainName
+    )).override
+      {
+        extensions = [
+          "rustc-dev"
+        ];
+      };
+
 in
 
 (craneLib.overrideToolchain toolchain).buildPackage ({