Increase reflector speed

http/install-common.sh:
Instead of `--age 12` use `--latest 20` as parameter to `reflector`
which brings down the mirrorlist creation from over two minutes to about
12s.

Fixes #122
5 jobs for issues/122 in 27 minutes and 27 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Lint
passed shellcheck #10285

00:00:11

passed shfmt #10286

00:00:08

 
  Build
passed build #10287

00:22:53

 
  Test
passed test-cloudimg-qemu #10289

00:04:21

passed test-vagrant-boxes-format #10288

00:00:32