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

module_build_service/config.py

@@ -301,7 +301,7 @@ class Config(object):
             'type': dict,
             'default': {
                 'git://pkgs.fedoraproject.org':
-                    ('fedpkg clone --anonymous $1',
+                    ('fedpkg clone --anonymous {}',
                      'fedpkg --release module sources'),
             },
             'desc': 'Mapping between dist-git and command to '},