(maint) Fix acceptance Rakefile to use mirror

Prior to this commit, the acceptance Rakefile would not make use of the
shim in-place to distinguish between our internal Git mirror and GitHub
(the repo naming conventions differ slightly). When trying to run
acceptance job against the internal Git mirror, the Rakefile would
use the incorrect repo URL / naming scheme.

This commit modifies `acceptance/Rakefile` to use build_giturl().
Merge pull request #788 from rji/maint/stable/acceptance-rakefile-fix

(maint) Fix acceptance Rakefile to use mirror