neovim: set meta.mainProgram

[Hoverbear]

Motivation for this change

Neovim's main executable is nvim. When running commands such as nix run nixpkgs#neovim users receive and error like:

$ nix run nixpkgs#neovim
error: unable to execute '/nix/store/j03scl0i8da1f2rlfaqdcpz8awzh0agk-neovim-0.4.4/bin/neovim': No such file or directory

After this patch, users experience nvim launching:

$ nix run .#neovim
# nvim opens!

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.

[grahamc]

Looks like a great idea. This will need a minor change to our meta validation code:

nixpkgs/pkgs/stdenv/generic/check-meta.nix

Line 205 in ddea086

description = str;

adding a new field named mainProgram of the type str.

[Hoverbear]

@grahamc Thanks for this advice! I introduced a PR doing that in #120692.

While it's in a separate PR as well, this requires it, so I've rebased this PR on top.