Draft: Add NixOS support to multiple SR

This MR depends on not-yet-merged previous work:

• https://lab.nexedi.com/nexedi/slapos.core/-/merge_requests/800: slapos.core's check_software has to be adapted to the output of ldd in NixOS.

• https://lab.nexedi.com/nexedi/slapos.core/-/merge_requests/803: slapos.core also has to be patched to always build its own zc.buildout to not use the one from the slapos nix package. This is not an issue on other systems as slapos.core is usually already installed using slapos

• https://lab.nexedi.com/nexedi/slapos.recipe.cmmi/-/merge_requests/21: On NixOS, a lot of libraries are getting installed to lib64/ instead of lib/, this change makes the post-install fix directly in slapos.recipe.cmmi. Once this is merged, 1d810284 can be reverted