cargo: switch to python3

pkgs/development/compilers/rust/cargo.nix

···
-{ stdenv, file, curl, pkgconfig, python, openssl, cmake, zlib
+{ stdenv, file, curl, pkgconfig, python3, openssl, cmake, zlib
 , makeWrapper, libiconv, cacert, rustPlatform, rustc, libgit2
 , CoreFoundation, Security
 }:
···
   dontUpdateAutotoolsGnuConfigScripts = true;
 
   nativeBuildInputs = [ pkgconfig cmake makeWrapper ];
-  buildInputs = [ cacert file curl python openssl zlib libgit2 ]
+  buildInputs = [ cacert file curl python3 openssl zlib libgit2 ]
     ++ stdenv.lib.optionals stdenv.isDarwin [ CoreFoundation Security libiconv ];
 
   LIBGIT2_SYS_USE_PKG_CONFIG = 1;