Fix: env rules merging for command line config (fixes #1271)

The rules for environments specified in a command line config file are now merged properly *before* any explicit rule from the same file.

I added one extra text for this particular behaviour. I'm not sure whether it would be more logical to rewrite the rest of the `config file` tests to check that the environment merging is done properly. It may not be worth it if the `-c` flag is to be dreprecated in the future.