Bluetooth: mesh: Temporarily allow K_FOREVER on system workqueue

[jori-nordic]

This is intended as a hotfix of sorts, in order to give some time to the Bluetooth Host and Mesh stack maintainers to figure out a sustainable solution.

This commit should be reverted when that better solution is found.

Hotfixes https://github.com/zephyrproject-rtos/zephyr/issues/77241 DO NOT close the issue when the PR is merged. We should still design a better long-term solution to this problem.