Make gem install --local not attempt remote access

gem install --local has been attempting remote access since
ccde1e0e2e4dba6a7e0951aab37b70b9a93429db was committed back
in February.  This is because it prefetches even in --local
mode.  Skip prefetching if --local is used, so that no remote
access happens in that case.

There were no tests committed when this bug was committed
(https://github.com/rubygems/rubygems/commit/ccde1e0e2e4dba6a7e0951aab37b70b9a93429db),
so hopefully none are needed to fix it.