kakoune: override 'version' env var

* kakoune: Prefix version with 'v' (#50617)

Turns out there's some tooling parsing the version number and having a
'v' prefix works better in that case.

I tested that it builds & works fine on NixOS.

See: https://github.com/ul/kak-lsp/issues/99

* Override 'version' env variable