Flutter has build tests that require iOS provisioning profiles (bundling apps for distribution, then validating that the bundle was built correctly) that do not required attached devices. Currently, the ios_signing dependency is only available on devicelab_machines. Make that dependency available to non-devicelab_machines so these tests can run in prod and don't take up scarce devicelab resources.

See https://github.com/flutter/infra/pull/288#discussion_r539835815