Use string interpolation to generate Cargo.lock path

[Zabot]

When specifying a custom workspaceSrc

pkgs.rustBuilder.makePackageSet {
  ...
  workspaceSrc = nix-filter {
    root = ./.;
    include = [
      "crates"
      ./Cargo.toml
      ./Cargo.lock
    ];
  };
}

nix build will fail with the following error

at /nix/store/yr2fm9jbvcvy4p55b6d1s4s5bx9pnhnz-source/Cargo.nix:40:21:

   39|   workspaceSrc = if args.workspaceSrc == null then ./. else args.workspaceSrc;
   40|   currentLockHash = builtins.hashFile "sha256" (workspaceSrc + /Cargo.lock);
     |                     ^
   41|   lockHashIgnored = if ignoreLockHash

error: access to absolute path '/Cargo.lock' is forbidden in pure eval mode (use '--impure' to override)

If you do build with --impure, the build later fails at the same place, this time failing to find the directory

at /nix/store/x2ysy1n9p12jrfk1s5kaxjkwm9q22kgl-source/Cargo.nix:40:21:

   39|   workspaceSrc = if args.workspaceSrc == null then ./. else args.workspaceSrc;
   40|   currentLockHash = builtins.hashFile "sha256" (workspaceSrc + /Cargo.lock);
     |                     ^
   41|   lockHashIgnored = if ignoreLockHash

error: getting status of '/Cargo.lock': No such file or directory

Both of these errors are because nix attempts to resolve the /Cargo.lock path used to lookup the lockfile as an absolute path on the file system, and fails. Instead we should use string interpolation to append the /Cargo.lock path to whatever path was passed in through the workspaceSrc.

I'm not quite sure why this isn't an issue when the workspaceSrc is not set. It seems like its related to how nix tries to evaluate the expression though, perhaps because the left hand side is unambiguously a path the right side is simply concatenated instead of being resolved first.

[psionic-k]

I think I just reviewed a duplicate change for this, but either way this change will be intergrated. Thanks.