Change Nightly build job to use git rather than svn

The nightly build points to the 7.7.x release branch

https://hudson.qa.jboss.com/hudson/view/SOA-Team/view/SOATools-3.3.0/job/soatools-3.3.x_Nightly.component--teiid-designer/

Given this release branch is being maintained in svn until the release of 8.0, the nightly build will continue to point to svn. Only on the release of 8.0, should this be reconfigured.