kakoune: restore the ability to find ambient programs

In e4da18a692, logic was added to the kakoune
wrapper to add a specific directory to the PATH. However, this accidentally
used the build-time value, thus breaking the ability of kakoune to find
programs on the system-wide PATH. Restore this ability by appending to the
run-time value.

pkgs/applications/editors/kakoune/wrapper.nix

@@ -24,7 +24,7 @@ in
       rm "$out/bin/kak"
       makeWrapper "${kakoune}/bin/kak" "$out/bin/kak" \
         --set KAKOUNE_RUNTIME "$out/share/kak" \
-        --set PATH "$PATH:$out/share/kak/bin"
+        --suffix PATH : "$out/share/kak/bin"
 
       # currently kakoune ignores doc files if they are symlinks, so workaround by
       # copying doc files over, so they become regular files...