Guard against ConcurrentModificationExceptions in the systemProperties PropertySource

[philwebb]

The StandardEnvironment currently adds a MapPropertySource containing System.getProperties() to allow them to be used from the Environment. Since the underlying Properties can be changed in other threads, it's possible that calls to propertySource.getPropertyNames() will throw a ConcurrentModificationException.

This can be seen in the following Spring Boot issue. Although we can add guards in the Spring Boot code, It might be useful if these exceptions could be handled transparently by the property source.

[philwebb]

Using synchronized alone might not be enough since it depends on every other access also adopting the same conventions. See spring-projects/spring-boot#17013 (comment)