Fix nimbus test using private Glean var

[skhamis]

Currently when trying to combine ios and ios-rust directories. There is a good oppurtunity to get rid of Glean bundled as a sub module and instead consume Glean as a swift package. Unfortunately, nimbus uses an internal variable of Glean that allows one of the tests to work properly with the swift package manager. This is a tracking task to follow up and fix that test after https://github.com/mozilla/application-services/pull/4830 lands.

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