### Description

Currently, the `airflow variables export` command requires an explicit file path and does not support output to stdout. However connections can be printed to stdout using `airflow connections export -`. This inconsistency between the two export commands can lead to confusion and limits the flexibility of the variables export command.

### Use case/motivation

To bring some consistency, similar to connections, variables should also be printed to STDOUT, using `-` instead of filename. 

### Related issues

_No response_

### Are you willing to submit a PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

if has_uri or has_json:
        invalid_args = []
        if has_uri and not _valid_uri(args.conn_uri):
            raise SystemExit(f"The URI provided to --conn-uri is invalid: {args.conn_uri}")
        for arg in alternative_conn_specs:
            if getattr(args, arg) is not None:
                invalid_args.append(arg)
        if has_json and args.conn_extra:
            invalid_args.append("--conn-extra")
        if invalid_args:
            raise SystemExit(
                "The following args are not compatible with "
                f"the --conn-{'uri' if has_uri else 'json'} flag: {invalid_args!r}"
            )
    if args.conn_uri:
        new_conn = Connection(conn_id=args.conn_id, description=args.conn_description, uri=args.conn_uri)
        if args.conn_extra is not None:
            new_conn.set_extra(args.conn_extra)
    elif args.conn_json:
        new_conn = Connection.from_json(conn_id=args.conn_id, value=args.conn_json)
        if not new_conn.conn_type:
            raise SystemExit("conn-json is invalid; must supply conn-type")
    else:
        new_conn = Connection(
            conn_id=args.conn_id,
            conn_type=args.conn_type,
            description=args.conn_description,
            host=args.conn_host,
            login=args.conn_login,
            password=args.conn_password,