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.

pkgs/build-support/rust/default.nix

@@ -4,7 +4,7 @@ let
     inherit stdenv cacert git rust cargo-vendor;
   };
 in
-{ name, cargoSha256 ? null
+{ name, cargoSha256 ? "unset"
 , src ? null
 , srcs ? null
 , sourceRoot ? null
@@ -17,7 +17,7 @@ in
 , cargoVendorDir ? null
 , ... } @ args:
 
-assert cargoVendorDir == null -> cargoSha256 != null;
+assert cargoVendorDir == null -> cargoSha256 != "unset";
 
 let
   lib = stdenv.lib;