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