Currently Debian sets /etc/debian_version to "bullseye/sid" and, due
to a series of issues explained in [1] more fully "lsb_release -c" in
the OpenDev environment doesn't return the distribution code name.
Overriding this to the final release version fixes this.
[1] http://lists.opendev.org/pipermail/service-discuss/2021-April/000222.html
Change-Id: I00c1741dac6ad5f2c4bf855a207f17d8985bc763
