fix /run/log directory test in miniserve pre-commands

This fixes a small issues that make a test fail (I've seen this on ubuntu):

2026-01-09 12:41:08 ::  [    8.851341] cloud-init[1364]: /var/lib/cloud/instance/scripts/runcmd: 102: [[: not found

The [[ is usable only with bash, and cloud-init, at least in some setups, uses sh.