cargo lambda deploy --env-file .env.foobar appears to succeed even if .env.foobar does not exist.

Suggest returning an error if path.is_file() is false:

https://github.com/cargo-lambda/cargo-lambda/blob/main/crates/cargo-lambda-metadata/src/env.rs#L69