[BUG] OSMC is not recognized as Debian-based

[nkuttler]

Description
OSMC is a Debian-based Linux distribution. As the os_family grain is not set correctly all apt-related things break, see also osmc/osmc#222

Setup
Unfortunately an OSMC install is needed to verify this. Then just try something like salt minion pkg.list_upgrades

Please be as specific as possible and give set-up details.

• on-prem machine
• VM (Virtualbox, KVM, etc. please specify)
• VM running on a cloud service, please be explicit and add details
• container (Kubernetes, Docker, containerd, etc. please specify)
• or a combination, please be explicit
• jails if it is FreeBSD

Versions Report

salt --versions-report

(Provided by running salt --versions-report. Please also mention any differences in master/minion versions.)

Salt Version:
          Salt: 3004.2
 
Dependency Versions:
          cffi: Not Installed
      cherrypy: Not Installed
      dateutil: 2.8.1
     docker-py: Not Installed
         gitdb: Not Installed
     gitpython: Not Installed
        Jinja2: 2.11.3
       libgit2: Not Installed
      M2Crypto: Not Installed
          Mako: Not Installed
       msgpack: 1.0.0
  msgpack-pure: Not Installed
  mysql-python: Not Installed
     pycparser: Not Installed
      pycrypto: Not Installed
  pycryptodome: 3.9.7
        pygit2: Not Installed
        Python: 3.9.2 (default, Feb 28 2021, 17:03:44)
  python-gnupg: Not Installed
        PyYAML: 5.3.1
         PyZMQ: 20.0.0
         smmap: Not Installed
       timelib: Not Installed
       Tornado: 4.5.3
           ZMQ: 4.3.4
 
System Versions:
          dist: debian 11 bullseye
        locale: utf-8
       machine: x86_64
       release: 5.10.0-15-amd64
        system: Linux
       version: Debian GNU/Linux 11 bullseye

Additional contextAdd any other context about the problem here
I'll create a pull request, the necessary change is trivial.