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

pkgs/applications/editors/kakoune/default.nix

@@ -21,6 +21,10 @@ stdenv.mkDerivation rec {
     sed -ie 's#--no-xmllint#--no-xmllint --xsltproc-opts="--nonet"#g' Makefile
   '';
 
+  preConfigure = ''
+    export version="v${version}"
+  '';
+
   meta = {
     homepage = http://kakoune.org/;
     description = "A vim inspired text editor";