buildRustCrate: fixup usage of builtins.filterSource

[andir]

Motivation for this change

While unifying most of the lib function calls I accidentially changed
the filterSource functions as well. Since there were no tests I ended
up forgetting about this case (even thought I ran into it…).

https://github.com/NixOS/nixpkgs/pull/75563/files/0aac0e8d2c9c129bf1ed868bdc735b738b17b44e#r363462659

@xzfc Can you verify that this solves your problem?

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

[andir]

@GrahamcOfBorg buildRustCrateTests

[xzfc]

@xzfc Can you verify that this solves your problem?

Yes, it is solved by this commit.