rust: Allow setting cargoSha256 to null.

Setting the hash to null is a convenient way to bypass the hash check
while developing. It looks like the ability to do this was inadvertently
removed while adding vendor directory support.

This still checks that the user is explicitly setting the value but
allows null as a valid option.

(cherry picked from commit 4499513e54dcd8c76280fca69083a139cacd0303)

pkgs/build-support/rust/default.nix

···
     inherit stdenv cacert git rust cargo-vendor;
   };
 in
-{ name, cargoSha256 ? null
+{ name, cargoSha256 ? "unset"
 , src ? null
 , srcs ? null
 , sourceRoot ? null
···
 , cargoVendorDir ? null
 , ... } @ args:
 
-assert cargoVendorDir == null -> cargoSha256 != null;
+assert cargoVendorDir == null -> cargoSha256 != "unset";
 
 let
   lib = stdenv.lib;