Make os.get_env consistent across Unixes
This matches the function name and API from env_windows.odin and os_linux.odin, which should be the same everywhere. Meaning:
* named get_env and not getenv
* return a string (empty if the environment variable is not found)
* accept a default value parameter for the allocator (defaulting to context.allocator)
* calls lookup_env which returns an extra found boolean value
This is so that you don't have to write platform/OS conditionals when getting environment variable values from the stdlib os.get_env/getenv function.
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