utils: postgres_admin_cmd chdir to / before running

utils: postgres_admin_cmd chdir to / before running

postgres_admin_cmd switches user to the postgres user.

When running a cli command in `/root` (or any other directory not
accessible by the postgres user) this causes:

`could not change directory to "/root": Permission denied`

to be printed multiple times on stderr for those invocations, which is
confusing and has caused quite a few support requests.

modifying the postgres_admin_cmd invocation only should not cause any
future surprises

quickly tested with `pmgconfig sync`

Reported-by: Oguz Bektas <o.bektas@proxmox.com>
Signed-off-by: Stoiko Ivanov <s.ivanov@proxmox.com>