allow install.sh to work offline

[mroch]

i'm trying to use this behind a firewall using a cache of the mirror, but install.sh hits github to make sure curl works and to calculate the mirror scheme.

when --quiet, it doesn't show the mirror URL anyway, so I'd propose moving the mirror stuff below this line:

if [ $be_quiet -eq 1 ]; then
    exit 0
fi

and perhaps find a different way to test whether curl is working that doesn't hit the network?