ci/build: limit debian-build-testing to 30 minutes

Previous 45 minutes is too much.

 - 5 - 15 minutes mesa LTO build
 - 5 - 15 minutes shader-db run

Should be safe, in case something fails, we still can make another run.

Signed-off-by: David Heidelberg <david.heidelberg@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24607>

.gitlab-ci/build/gitlab-ci.yml

@@ -203,6 +203,7 @@ debian-build-testing:
     .gitlab-ci/meson/build.sh
     section_switch shader-db "shader-db"
     .gitlab-ci/run-shader-db.sh
+  timeout: 30m
 
 # Test a release build with -Werror so new warnings don't sneak in.
 debian-release: