get rid of weird timedatectl error

[maxheld83]

System has not been booted with systemd as init system (PID 1). Can't operate.
> Failed to create bus connection: Host is down
> Warning message:
> In system("timedatectl", intern = TRUE) :
>   running command 'timedatectl' had status 1

[maxheld83]

happens on github actions as per f00feb0

[jimhester]

You should be able to avoid this by setting the TZ environment variable explicitly e.g. rocker-org/rocker-versioned#89