Add sdk user to the sudo group to fix blueman regression

The user account `user` that is created on the SDK already has sudo
powers. However this user account is not part of the sudo group.

With the upgrade of blueman from 2.0.8-1co1 to 2.0.8-1+deb10u1co1
this has was now causing an error dialog saying that
"You might not be able to connect to the Bluetooth network via this machine":

https://github.com/blueman-project/blueman/issues/948



It seems that polkit was disabled and the update which came from
debian-security to fix CVE-2020-15238 enabled it:

> Blueman is a GTK+ Bluetooth Manager. In Blueman before 2.1.4, the
> DhcpClient method of the D-Bus interface to blueman-mechanism is prone
> to an argument injection vulnerability. The impact highly depends on the
> system configuration. If Polkit-1 is disabled and for versions lower
> than 2.0.6, any local user can possibly exploit this. If Polkit-1 is
> enabled for version 2.0.6 and later, a possible attacker needs to be
> allowed to use the `org.blueman.dhcp.client` action. That is limited to
> users in the wheel group in the shipped rules file that do have the
> privileges anyway. On systems with ISC DHCP client (dhclient), attackers
> can pass arguments to `ip link` with the interface name that can e.g. be
> used to bring down an interface or add an arbitrary XDP/BPF program. On
> systems with dhcpcd and without ISC DHCP client, attackers can even run
> arbitrary scripts by passing `-c/path/to/script` as an interface name.
> Patches are included in 2.1.4 and master that change the DhcpClient
> D-Bus method(s) to accept BlueZ network object paths instead of network
> interface names. A backport to 2.0(.8) is also available. As a
> workaround, make sure that Polkit-1-support is enabled and limit
> privileges for the `org.blueman.dhcp.client` action to users that are
> able to run arbitrary commands as root anyway in
> /usr/share/polkit-1/rules.d/blueman.rules.

With polkit now being enabled, this snippets becomes relevant:

```
$ sudo cat /usr/share/polkit-1/rules.d/blueman.rules
// Allow users in sudo or netdev group to use blueman feature requiring root without authentication
polkit.addRule(function(action, subject) {
    if ((action.id == "org.blueman.network.setup" ||
         action.id == "org.blueman.dhcp.client" ||
         action.id == "org.blueman.rfkill.setstate" ||
         action.id == "org.blueman.pppd.pppconnect") &&
        subject.local && subject.active &&
        (subject.isInGroup("sudo") || subject.isInGroup("netdev"))) {
        return polkit.Result.YES;
    }
});
```

Adding the user to the `sudo` group would then make blueman happy again.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>