Use only POSIX commands and options

The hard-coded use of /bin/bash and of long options for nl does only
work on Linux based systems with GNU textutils.

If POSIX commands and options are used, this test becomes portable to
other systems than Linux.

tests/test_expect.py

diff --git a/tests/test_expect.py b/tests/test_expect.py
index c37e159..243bc34 100755
--- a/tests/test_expect.py
+++ b/tests/test_expect.py
@@ -411,7 +411,7 @@
     def test_before_across_chunks(self):
         # https://github.com/pexpect/pexpect/issues/478
         child = pexpect.spawn(
-            '''/bin/bash -c "openssl rand -base64 {} 2>/dev/null | head -500 | nl --number-format=rz --number-width=5 2>&1 ; echo 'PATTERN!!!'"'''.format(1024 * 1024 * 2),
+            '''/bin/sh -c "openssl rand -base64 {} 2>/dev/null | head -500 | nl -n rz -w 5 2>&1 ; echo 'PATTERN!!!'"'''.format(1024 * 1024 * 2),
             searchwindowsize=128
         )
         child.expect(['PATTERN'])