SystemEnvironmentPropertySource is no longer considered immutable

[philwebb]

We have detection logic in SpringIterableConfigurationPropertySource which treats the SystemEnvironmentPropertySource as immutable.

This logic has existed for a while, but with the introduction of OriginAwareSystemEnvironmentPropertySource it no longer applies sine the earlier OriginLookup check happens. Technically this is probably a regression, but since the impact is minimal we should probably only fix it in the next minor.