selftests: mptcp: set all env vars as local ones

It would be better to move the declaration of all the env variables to
do_transfer(), run_tests(), or pm_nl_set_endpoint() as local variables,
instead of exporting them globally at the beginning of the file.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Link: https://lore.kernel.org/r/20230712-upstream-net-next-20230712-selftests-mptcp-use-local-env-v1-1-f1c8b62fbf95@tessares.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>