scripts/download.pl: fix mirrors regression for curl and wget

With the introduction of aria2c support, curl and wget no longer try to
download the file from mirrors. Fix this regression by emptying the
remaining mirrors list only when aria2c is used.

Fixes: d39123626931 ("download.pl: add aria2c support")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>