MockModuleBuilder: Fix thread safety problem with distgit-clone-wrapper
Multiple build threads would try to write to the same distgit-clone-wrapper
location, which could result in ETXTBSY when one thread tried to execute
a script that another was writing to or vice-versa.
Instead of using a wrapper script, use a sh -c invocation for distgit_get.
https://pagure.io/fm-orchestrator/issue/592