Environment variables starting with `BEGIN_` should not pass validation

[macdonst]

The CLI/API should not allow users to set environment variables that start with BEGIN_.

[ryanblock]

This should strictly be an API side validation error, although I’m open to relaxing this rule if we feel there is cause to allow it. We will need to have some reserved / internal env var namespace, though!

[macdonst]

@ryanblock yup, I realize the fix will lie in the API code but I wanted to make sure we test it from the CLI to return a good error message.