Bug 1071568 - Only pass down templates to the sandbox context when calling moz.build templates. r=mshal

It's not entirely clear passing down all the metadata makes sense. On the
other hand, when creating the template execution sandbox, passing down
exports does assign the value for the exported variable in that execution
context. When that context is merged with the caller sandbox context, the
exported variable is reassigned, even if the value is not modified. Then,
if the caller sandbox itself reassigns the exported variable, it fails
because calling a template already did it once, unexpectedly.

Not passing down exported variables makes the template execution sandbox
never set those exported variables, so that they are not merged back. The
caller sandbox can then properly reassign the exported variable.