Use a prefix for the STANDALONE environment variable

Seems like "STANDALONE" is too generic and causes a conflict in
certain environments (see bug link below). Add a prefix here to
decrease the possibility of a conflict here.

Resolves: https://gitlab.com/kvm-unit-tests/kvm-unit-tests/-/issues/3


Message-Id: <20220120182200.152835-1-thuth@redhat.com>
Acked-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>