modem/q6voiced: fix permission issue

Due to recent changes in https://gitlab.alpinelinux.org/alpine/aports/-/commit/f55297ecf3bff5e236bb6d536651742022a3dc03, running q6voiced as part of only audio group is no longer sufficient based on whether elogind is installed or not. Created a q6voiced user which is part of both audio and plugdev groups and run the service with that user for now.

Closes https://gitlab.com/postmarketOS/pmaports/-/issues/1802