buildRustPackage: Get rid of /proc/self/cwd hack

This makes buildRustPackage portable to non-Linux platforms.

Additionally, now we also save the `Cargo.lock` file into the fetch output, so
that we don't have to run $cargoUpdateHook again just before building.

pkgs/development/tools/build-managers/cargo/default.nix

@@ -23,7 +23,7 @@ buildRustPackage rec {
     cargo update -p threadpool --precise 0.1.4
   '';
 
-  depsSha256 = "12d2v4b85qabagrypvqiam2iybd4jwcg0sky0gqarfhjh2dhwfm6";
+  depsSha256 = "1gj3mnjj17h5p0r1jcm3m3pm3p3l1rbfdz3l7v1cykng78dsabnq";
 
   buildInputs = [ file curl pkgconfig python openssl cmake zlib ];