git-gui: massage "git-gui--askyesno" with "generate-script.sh"

git-gui: massage "git-gui--askyesno" with "generate-script.sh"

In e749c87 (git-gui: provide question helper for retry fallback on
Windows, 2025-08-28) we have introudced a new "git-gui--askyesno" helper
script. While the script is conceptually similar to our existing helper
script "git-gui--askpass", we don't massage it via "generate-script.sh".
This means that build options like the path to the wish shell are not
propagated correctly.

Fix this issue.

Signed-off-by: Patrick Steinhardt <ps@pks.im>