Add TZ envvar to default chart values

[npelikan]

When attaching lubridate, users will often see an error like:

> library(lubridate)
System has not been booted with systemd as init system (PID 1). Can't operate.
Failed to connect to bus: Host is down

Warning message:
In system("timedatectl", intern = TRUE): 
  running command 'timedatectl' had status 1

Investigation reveals this is related to rocker-org/rocker-versioned#242

I'd suggest we set the TZ envvar to UTC by default in PWB and PC containers so users don't see this confusing error.

[fh-mthomson]

FYI this is now set by default in the upstream containers: rstudio/rstudio-docker-products#725

[npelikan]

Closing as its resolved upstream!