Correctly determine win arch for installing packages.

My previous implementation had a bug in that it expected execute to
return a result object.  Instead, execute exposes the result object to a
passed block.  So now we check the result within a block and set rootdir
and cygwin executable there.
Merge pull request #47 from jpartlow/issue/master/windows_package_install_fails_testing_result

Correctly determine win arch for installing packages.