Codecov: don't post status until 2 builds received

We currently upload unit test coverage and UI test coverage separately.
This tells Codecov to not compute/notify coverage until both are received.

This _should_ prevent the flopping coverage when one of the test suites fails,
or when CI is skipped (due to no code changes, for example, for Drone)

Signed-off-by: Álvaro Brey <alvaro.brey@nextcloud.com>

.codecov.yml

@@ -2,6 +2,8 @@ codecov:
   branch: master
   ci:
     - drone.nextcloud.com
+  notify:
+    after_n_builds: 2
 
 coverage:
   precision: 2