Dont check prefixes when not running from one of the prefix.

The check prefixes function only makes sense, when we are running the
enpkg tool.  But, enstaller can be used as a library, and an Enpkg
instance can be used to install packages into prefixes separate from
the one where the code is running.  In such a case, the checks
performed by this function are meaningless.

@tonysyu, can you look at this, since you added this check?  Thanks!