Fix test_undecodable_env of test_subproces for non-ASCII directory

Fix test_undecodable_env of test_subproces for non-ASCII directory

This test was introduced by r80421 (issue #8391).

The fix: copy the environment variables instead of starting Python in an empty
environement. In an empty environment, the locale is C and Python uses ASCII
for the default file system encoding. The non-ASCII directory will be encoded
using surrogates, but Python3 is unable to load a module or package with a
filename using surrogates.

See issue #8242 for more information about running Python3 with a non-ascii
directory in an empty environement.