selftests/nolibc: chdir_root: restore current path after test

The PWD environment variable has the path of the nolibc-test program,
the current path must be the same as it, otherwise, the test cases will
fail with relative path (e.g. ./nolibc-test).

Since only chdir_root really changes the current path, let's restore it
with the PWD environment variable.

Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>