The VM runtime scripts sourced pins.env optionally and duplicated the
LIBKRUN_REF and UMOCI_VERSION defaults, so every bump had to update the
same value in several files.
Require pins.env, drop the duplicated defaults, and fail on an invalid
LIBKRUN_REF instead of silently building upstream HEAD. Environment
overrides still take precedence through the defaults in pins.env.
Signed-off-by: Florent Benoit <fbenoit@redhat.com>