Actual mutability of the Unix process environment is something we have to give up when moving to multi-threaded processes. The only time you get to change the environment is when running single-threaded, typically around fork/exec. Learn to live with it.
The Java System.getenv() is initialized from the Unix environment and is also immutable. One could imagine making that mutable in the style of ConcurrentHashMap and having subprocesses inherit it, but it would open up a divergence between Java and native. It's easier for an environment like gradle to do the same sort of thing, but then there's the same problem - all the users would need to use exclusively gradle APIs to access the environment and spawn subprocesses, else the mismatch will cause trouble.
