tzsetup: symlink /etc/localtime instead of copying

[emaste]

Summary

Ref twitter thread at https://twitter.com/kaevans91/status/1580652408861958144

[philip]

Looks good to me. Thank you!

[jrm]

A key argument for this change is that we align with what most Linux distributions are doing. Since Linux/systemd says it should be a symbolic link, some third-party software like Chromium or R runs into problems when the zoneinfo file is copied to /etc/. The arguments I've read against this change (https://twitter.com/kaevans91/status/1580652408861958144) describe (theoretical) problems when /usr isn't mounted. However, peter@ says,

"I’ve never found a case where a symlink doesn’t Just Work."

[philip]

I don't see any reason not to do this. @emaste: do you want to commit this, or shall I?

[emaste]

I'll push it later today or tomorrow