Skip slow unattended-upgrades install, ensure postgres can chdir

[yanokwa]

• Ensure postgres can chdir or you get a "could not change directory to "/root": Permission denied" error. See https://stackoverflow.com/questions/35782366 for alternatives.
• Do not run unattended-upgrades because it's very slow (especially on DO) and blocks the rest of the install. The OS will run it eventually.

The PR itself doesn't require a new release (which is great). And it's relatively safe because if installs fail, we'll likely get support requests and we can respond then.

I did a search for "unattended-upgrade" and "postgres -c" to make sure I didn't miss anything.

[ggalmazor]

(Apologies! will have some cycles for this during the weekend)