Fix CocoaPods ruby dependency for flavors_test_ios on Mac bot - #190775

This PR adds the missing ruby (CocoaPods) dependency to the flavors_test_ios target in .ci.yaml.
Following the migration of this test from the physical device pool (Mac_ios) to the generic Mac simulator pool (in PR #189442), the target began failing in post-submit bringup with the error CocoaPods not installed or not in valid state.
Because the dummy flavors app uses the integration_test package and contains an ios/Podfile, it requires CocoaPods to compile. Generic Mac CI bots do not have CocoaPods installed globally, so it must be explicitly requested in the LUCI .ci.yaml dependencies list.

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Fixes #189442

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