This makes it possible to not override previously defined environment
variables when running `dotenv run`.  It defaults to `--override` for
compatibility with the previous behavior.

The `get` subcommand would return `key=value`, which is impractical to
retrieve the value of a key in a script.  Since the `key` is already
known by the caller, there is no point in showing it.

This also makes the output consistent with the documentation for the
subcommand.