test/py: fix printenv signon message disable code

CONFIG_VERSION_VARIABLE isn't always defined, so we can't simply look up
its value directly, or an exception will occur if it isn't defined.
Instead, we must use .get() to supply a default value if the variable
isn't defined.

Fixes: da37f006 ("tests: py: disable main_signon check for printenv cmd")
Acked-by: Heiko Schocher <hs@denx.de>
Signed-off-by: Stephen Warren <swarren@nvidia.com>