Bug#969329: systemd-cron: Special user nobody configured, this is not safe!

[Pontus Stenetorp]

This should be the commit that introduced the warning:

    https://github.com/systemd/systemd/commit/bed0b7dfc0070e920d00c89d9a4fd4db8d974cf0

Based on reading it, setting `DynamicUser=yes` seems to be what systemd wants you to do.

Official documentation for `DynamicUser`:

    https://www.freedesktop.org/software/systemd/man/systemd.exec.html#DynamicUser=