gitignore: ignore all driver build dirs (build4_*)

The driver builds into build4_$(date +%Y%m%d_%H%M%S)_$; the old
build4_2* pattern only covered current-year timestamps. Ignore the
whole family so build artifacts stop showing up as untracked.

Co-Authored-By: Claude Code <noreply@anthropic.com>

qt6-after/qt6-migrated-legatus/.gitignore

@@ -14,4 +14,5 @@ build/
 build2/
 .envrc
 build4
+build4_*
 deb_build_tmp