Move Linux coverage back to bringup (flutter#174171)

Because this only runs every 6 commits, it being flaky takes a long time
to recover.

It's not a priority to keep green and un-flaky at the moment.

Author: matanlurey

.ci.yaml

@@ -375,6 +375,8 @@ targets:
       task_name: analyzer_benchmark
 
   - name: Linux coverage
+    # Because this only runs every 6 commits, it being flaky takes a long time to recover.
+    bringup: true 
     presubmit: false
     recipe: flutter/coverage
     timeout: 90