Repair software release tests

* [slapos-master](https://nexedijs.erp5.net/#/test_result_module/20210629-DE509CB3/23) is failing in checkSoftware step, because:
  * wcfs is linked against system zlib (fixed in this MR)
  * the .so in the build directory have missing references. I think we don't need to check in build folders and also liblibgolang is missing, but it's not a problem because of the way we load dynamically at runtime, so I suggest changing checkSoftware in slapos.core!317
* [gitlab](https://nexedijs.erp5.net/#/test_result_module/20210629-DE509CB3/10) is failing for some time because mimemagic have been yanked from rubygems. This is a minimal change to get the test running again, to prevent other problems to sneak in while we don't have tests

See merge request !1007