ci: Don't run cargo update on the stable branch

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/merge_requests/1091>

.gitlab-ci.yml

@@ -72,13 +72,6 @@ trigger:
   before_script:
     - source ./ci/env.sh
     - mkdir .cargo && echo -e "[net]\ngit-fetch-with-cli = true" > .cargo/config
-    # If cargo exists assume we probably will want to update
-    # the lockfile
-    - |
-      if command -v cargo; then
-        cargo generate-lockfile
-        cargo update
-      fi
 
 .debian:11-stable:
   extends: .debian:11