[shellenv] PrintEnv should generate just in case

[mikeland73]

Summary

Fixes #829

run and shell always call generate but using shell --print-env and shellenv don't. This means that sometimes the .devbox directory might be out of date (e.g. when we turned on flakes, not everyone has flake.nix file). Since not all workflows involve shell and run, some users never update it. For performance reasons, we only update if the devbox version doesn't match whatever version created the .devbox dir.

This change may not be sufficient to fix all cases, but it fixes the attached issue.

How was it tested?

rm .devbox/gen/flake/flake.nix
devbox shellenv > /dev/null
rm .devbox/gen/flake/flake.nix
devbox shell --print-env > /dev/nul
``

no error (previously this gave me same error as issue)