rustPlatform.fetchCargoVendor: split script into two parts

[TomaSajt]

A POC for a possible way to split fetchCargoVendor's util script into two different util scirpts.

This way, changing the FOD util script will not cause it to rebuild the non-FOD part, thus avoiding mass rebuilds.

Currently there's one pending change to the FOD part: #377632
Another would be to support overriding the fetching url to fetch from a different mirror. This should be easy, but I haven't implemented it yet. (#377558)

Note: by "the FOD changing" I mean its implementation, not the result (and thus, the hash).

One thing to note is that optimally, the FOD part shouldn't really change a lot. And if we change the non-FOD part, only the non-FOD stuff will get rebuilt (since FODs don't rebuild unless their name changes).

So the question is: is it even worth separating this out into multiple files.
If yes, what's a better way of doing it?

I'll keep this PR as a draft until we decide what's the best way to split it up.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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