vsdownload: Fix unable to set --only-host to False

[huangqinjin]

Previously --only-host didn't accept any argument. Now user can explicitly specify yes or no.

$ ./vsdownload.py -h
--only-host [yes|no]  Only download packages that match host arch (default: yes)

[mstorsjo]

This looks good to me, although my Python is a bit too rusty to follow the details of how the custom Action works here.