rustc: put targetPrefix in pname

Our rustc package is not universal, because we only build std for the
host and target platforms. This means that a build graph where cross
is involved will end up with multiple rustc packages in it, so it
would be helpful to have a way to tell them apart, just like we do for
e.g. gcc.

pkgs/development/compilers/rust/rustc.nix

···
   inherit (lib) optionals optional optionalString concatStringsSep;
   inherit (darwin.apple_sdk.frameworks) Security;
 in stdenv.mkDerivation rec {
-  pname = "rustc";
+  pname = "${pkgsBuildTarget.targetPackages.stdenv.cc.targetPrefix}rustc";
   inherit version;
 
   src = fetchurl {