run-unit-test-docker.sh: Make capitalisation of DISTRO consistent
run-unit-test-docker.sh: Make capitalisation of DISTRO consistent

Previously the script required $distro be set in the environment, but
used $DISTRO internally. This is confusing, so require the capitalised
version always.

Hopefully this won't break any external scripts.

Tested: Ran /bin/bash -n on the script, parsed without failure.

Change-Id: I64eea6638c3a9a28138495f7b849373486323d6f
Signed-off-by: Andrew Jeffery <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@aj.id.au>

openbmc/openbmc-build-scripts,openbmc/openbmc-build-scripts