Not currently overriding existing environment variables when using envfile

[kylerc]

Right now if you specify envfile when calling a local lambda, that file's variables do not override existing environment variables that are already set. For our use case, overriding would be preferable and also that had been our initial expectation for how the envfile setting would work which was not the case. Would be nice if this could either change or expose an option to opt into overriding existing environment variables.

We were able to work around this issue in our usecase by loading the desired environment file and passing it in as an object to the environment when calling lambda-local which does override the existing environment variables, but those behaving differently seems unexpected.