Move linux_fuchsia to staging (#50288)

For https://github.com/flutter/flutter/issues/142811
diff --git a/.ci.yaml b/.ci.yaml
index edc4faa..a893adb 100644
--- a/.ci.yaml
+++ b/.ci.yaml
@@ -201,6 +201,7 @@
       - os=Linux
     dimensions:
       kvm: "1"
+    bringup: true
     # TODO(https://github.com/flutter/flutter/issues/138559): Re-enable/delete.
     # runIfNot:
     #   - lib/web_ui/**