gnvim: fix install phase with Rust with custom target

[minijackson]

Motivation for this change

When building with cargo build --release --target <arch> ..., Rust installs the gnvim binary in ./taget/<arch>/release instead of ./target/release, which isn't taken into account by upstream's Makefile.

Normally, the project would have been compiled by using make build, but if I understood it correctly, the buildPhase is now overridden by the new hook system (#112804)

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.

[r-rmcgibbo]

Result of nixpkgs-review pr 115267 at ec5fb3c run on x86_64-linux 1

2 packages failed to build:

• gnvim
• gnvim-unwrapped

[minijackson]

@ofborg build gnvim

[teto]

$ git worktree add /home/teto/.cache/nixpkgs-review/pr-115267/nixpkgs 7d15e227cf3b30cddd083370814b8d1a2f05ed49
Preparing worktree (detached HEAD 7d15e227cf3)
Updating files: 100% (24774/24774), done.
HEAD is now at 7d15e227cf3 Merge pull request #115232 from xantoz/nyxt-more-platforms
$ git merge --no-commit ec5fb3cce8c8827575a1f2315e3a43b5be55d23c
Automatic merge went well; stopped before committing as requested
$ nix --experimental-features nix-command build --no-link --keep-going --option build-use-sandbox relaxed -f /home/teto/.cache/nixpkgs-review/pr-115267/build.nix

Link to currently reviewing PR:
https://github.com/NixOS/nixpkgs/pull/115267

2 packages built:
gnvim gnvim-unwrapped

it took a while but build passes locally. Binary works.