bpo-33455: Pass os.environ in test_posix::test_specify_environment. (GH-6753)

bpo-33455: Pass os.environ in test_posix::test_specify_environment. (GH-6753)

Pass os.environ's copy to new process created at test_posix:
test_specify_environment. Otherwise important variables such as
LD_LIBRARY_PATH are not set and the child process might not work at all
in an environment where such variables are required for Python to function.