update-alternatives.bbclass: don't expand var when appending do_install
Or else absolute paths may be expanded which then breaks sstate signature
comparison.
I'm not sure whether pre/post funcs can be expanded or not. They are
invoked at image build or on the target, and thus need be expanded some
place. But if one recipe does use directories under TMP in its postinst
helpers, this would be an issue again.
