rust: 1.17.0 -> 1.20.0

simplify the boostrap hashes a bit

build with bundled llvm:

the rust project has forked the llvm compiler to solve some
issues.
With pkgs.llvm the test suite fails.

See https://github.com/rust-lang/rust/pull/43026

And PR #30088

pkgs/development/compilers/rust/cargo.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchgit, file, curl, pkgconfig, python, openssl, cmake, zlib
 , makeWrapper, libiconv, cacert, rustPlatform, rustc, libgit2
-, version, srcRev, srcSha, depsSha256
+, version, srcSha, depsSha256
 , patches ? []}:
 
 rustPlatform.buildRustPackage rec {
@@ -9,7 +9,7 @@ rustPlatform.buildRustPackage rec {
 
   src = fetchgit {
     url = "https://github.com/rust-lang/cargo";
-    rev = srcRev;
+    rev = version;
     sha256 = srcSha;
   };