Revert "testing/check_software: no longer tolerate system libnsl"

This reverts commit 2d50c8b8.

We are not ready for this, for example on debian 10 perl link with
libnsl.

For reference, one example test failure:
  https://erp5js.nexedi.net/#/test_result_module/20220111-190A520F2/20

We'll try to re-apply this later once no software use system libnsl

slapos/testing/check_software.py

@@ -68,6 +68,7 @@ def checkSoftware(slap, software_url):
       'libgcc_s',
       'libgomp',
       'libm',
+      'libnsl',
       'libpthread',
       'libthread_db',
       'libresolv',