Use 5 shards for macOS presubmits.

This should get us below 10 minutes, hopefully :)

RELNOTES: None.
PiperOrigin-RevId: 295713685
diff --git a/.bazelci/presubmit.yml b/.bazelci/presubmit.yml
index 78535c9..8cd8c48 100644
--- a/.bazelci/presubmit.yml
+++ b/.bazelci/presubmit.yml
@@ -131,7 +131,7 @@
     test_targets:
       - "//src/test/shell/bazel:cc_integration_test"
   macos:
-    shards: 4
+    shards: 5
     shell_commands:
       - sed -i.bak -e 's/^# android_sdk_repository/android_sdk_repository/' -e 's/^#
         android_ndk_repository/android_ndk_repository/' WORKSPACE