do-release-upgrade: preserve env vars needed for screen lock prevention

The screen lock prevention code relies on environment variables such as
DBUS_SESSION_BUS_ADDRESS being preserved, since the
org.freedesktop.ScreenSaver DBus interface is needed. This works in
cases where the dist-upgrade script is invoked with 'sudo -E' (e.g.
DistUpgradeViewText frontend). However, when pkexec is used (e.g.
DistUpgradeViewGtk3 frontend) the environment variables cannot be
preserved as per pkexec's design.

Add an --env flag to do-release-upgrade that accepts a comma-separated
list of environment variables (e.g. VAR1=VALUE1,VAR2=VALUE2) which should be set when
executing the dist-upgrade script. This approach is similar to the
existing workaround for RELEASE_UPGRADER_ALLOW_THIRD_PARTY, but is
generalized to allow any environment variables to be set.

Finally, when the DistUpgradeViewGtk3 or DistUpgradeViewKDE frontends
are used, and the do-release-upgrade script is re-exec'd with pkexec,
set the DBUS_SESSION_BUS_ADDRESS and XDG_SESSION_TYPE variables with
--env so that the screen lock prevention code works with these
frontends.

LP: #1968607