[gnome-system-monitor] Update .gitlab-ci.yml to fix ubuntu ci build.

[Robert Roth <robertroth src gnome org>]

commit a4d5bd6688a2e08ecdad4185754f212c42bc28e5
Author: Robert Roth <robert roth off gmail com>
Date:   Sun Mar 6 09:57:02 2022 +0000

    Update .gitlab-ci.yml to fix ubuntu ci build.

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 219e82b1..b9769b90 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -67,7 +67,7 @@ build-fedora:
     - ninja -C _build install
 
 build-ubuntu:
-  image: ubuntu:latest
+  image: ubuntu:rolling
   stage: build
   except:
     - tags