Summary:
Fix the BuildOptions class to correctly honor the `host_type` parameter that
it was constructed with when constructing the manifest evaluation context.
I accidentally broke this behavior in D16477396, and incorrectly had this code
path default to using the current host system rather than the value passed in
from the command line.

Reviewed By: wez

Differential Revision: D16779579

fbshipit-source-id: de911daaa643f6303fd35149775ab25d3f64d34f