fix #1556: dub --cache=local fetch + build ...

Building a package that was locally cached failed, because
the dependencies whould be fetched at the wrong destination.

For example if one tries to build the package 'optional':

$ dub --cache=local fetch optional
-> optional is now at ./.dub/package/optional
$ dub --cache=local build optional
-> fails because the dependency bolts was fetched to
./.dub/package/optional/.dub/package/bolts.

This is fixed by not overwriting the root path if dub builds
a package from the cache.