[env] Add --env and --env-file flags

[mikeland73]

Summary

Adds --env and --env-file flags that can be used with run shell and services

If both flags are specified they are merged with --env taking precedence.

environment variables passed in with flags take the highest level of precedence, replacing any env variables set by nix, devbox.json, or plugins.

TODO: The flag code is a bit brittle and repetitive, I think we can consolidate the common flag registration.

How was it tested?

devbox run -e FOO=BAR echo \$FOO
devbox services up -e HTTPD_PORT=8083

[mohsenari]

We should add in our docs that if both env and env-file are specified env takes priority

[Lagoja]

Should also help with #1215, since we can set the NIX_HARDENING_ENABLE variable with the -e flag

[sentry-io]

Suspect Issues

This pull request was deployed and Sentry observed the following issues:

• ‼️ **syscall.Errno: error running command in Devbox: <redacted errors.withStack>: <redacted fs.PathError>: <redacte... go.jetpack.io/devbox/internal/impl in (*Devbox)... View Issue
• ‼️ **syscall.Errno: <redacted errors.withStack>: <redacted fs.PathError>: go.jetpack.io/devbox/internal/boxcli in (*envFl... View Issue

_{Did you find this useful? React with a 👍 or 👎}