archbuild: Turn off Git's safe.directory

Without this setting, Git exits with an error when the repository is not
owned by the current user. This messes with our shared srcdest.

roles/archbuild/files/gitconfig

+# vim:set ft=gitconfig noet sw=0 sts=-1:
+
+[safe]
+	directory = *

roles/archbuild/tasks/main.yml

@@ -115,3 +115,6 @@
 
 - name: install archbuild sudoers config
   copy: src=sudoers dest=/etc/sudoers.d/archbuild owner=root group=root mode=0440
+
+- name: install gitconfig
+  copy: src=gitconfig dest=/etc/gitconfig owner=root group=root mode=0644