cmd/snap-confine: chown private /tmp parent to root.root

When snap-confine creates a private /tmp directory for a given snap it
first creates a temporary directory in /tmp/ named after the snap, along
with a random name. Inside that directory it creates a /tmp directory
with permissions appropriate for a future /tmp, namely 1777.

Up until recently the that directory was owned by the user who first
invoked snap-confine. Since the directory is reused by all the users on
the system this logic makes no sense.

This patch changes the related logic so that the private /tmp directory
is owned by root, just like the real one.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>