Changes

sccp: testenv: fix hardcoded sccp_demo_user path

The current path only worked for with podman and with osmo-dev. Make it
work for the following use cases too:

* without podman, with osmo-dev
* with podman, with binary packages (instead of osmo-dev)

Removing package=no is required, so testenv build sccp_demo_user from
source when running with --binary-packages. This is needed as
sccp_demo_user is not packaged (OS#5899).

Change-Id: Id6e0b669dcb71cab9e789fe011e60827a8e07f77