Allow configuring the nvim path used inside s:inverse_search_cmd

[poscat0x04]

On NixOS, if one chooses to configure neovim declaratively, then the neovim binary gets wrapped inside a wrapper.
What the wrapper does is it calls the neovim binary with suitable arguments and environment variables. One of the arguments passed is -u /nix/store/h97rm0ckx2p9vqcc5hsdkskykw882j87-init.vim which is how the plugins such as vimtex get loaded.
This usually works fine, but when vimtex uses zathura for viewing pdf, and since get(v:, 'progpath') returns the path to the unwrapped binary, the unwrapped binary gets passed to zathura as part of the synctex command. This immediately causes a problem: since the unwrapped binary can't possibly load vimtex, the command will always fail.

One way to solve this is issue to allow configuring the nvim path used inside s:inverse_search_cmd so people using NixOS (and possibly GuixSD) can manually set the nvim path to the wrapper. Unfortunately, I'm don't know much vimscript so I cannot implement this.
See also: NixOS/nixpkgs#78033

[lervag]

This issue is discussed further in #2262.