t/t1517: indicate that gui--askyesno fails with GIT_TEST_INSTALLED

Commit e749c87 (git-gui: provide question helper for retry fallback
on Windows, 2025-08-28) causes `git-gui--askyesno` to be installed when
running `make install`. This is triggering a new occurrence of the
issues noted in commit c4cf8ca (t/t1517: mark tests that fail with
GIT_TEST_INSTALLED, 2025-08-19), which is preventing Git v2.52.0 from
building in nixpkgs [1]. Indicate that this is an expected failure.

[1]: NixOS/nixpkgs#465178 (comment)

Author: zivarah

t/t1517-outside-repo.sh

@@ -115,7 +115,7 @@ do
 	credential-osxkeychain | cvsexportcommit | cvsimport | cvsserver | \
 	daemon | \
 	difftool--helper | filter-branch | fsck-objects | get-tar-commit-id | \
-	gui | gui--askpass | \
+	gui | gui--askpass | gui--askyesno | \
 	http-backend | http-fetch | http-push | init-db | \
 	merge-octopus | merge-one-file | merge-resolve | mergetool | \
 	mktag | p4 | p4.py | pickaxe | remote-ftp | remote-ftps | \