### 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)

return fileio.name == "<stdout>"
def _valid_uri(uri: str) -> bool:
    """Check if a URI is valid, by checking if scheme (conn_type) provided."""
    return urlsplit(uri).scheme != ""
@cache
def _get_connection_types() -> list[str]:
    """Returns connection types available."""
    _connection_types = []
    providers_manager = ProvidersManager()
    for connection_type, provider_info in providers_manager.hooks.items():
        if provider_info:
            _connection_types.append(connection_type)
    return _connection_types
@providers_configuration_loaded
def connections_export(args):
    """Exports all connections to a file."""
    file_formats = [".yaml", ".json", ".env"]
    if args.format:
        warnings.warn("Option `--format` is deprecated.  Use `--file-format` instead.", DeprecationWarning)
    if args.format and args.file_format:
        raise SystemExit("Option `--format` is deprecated.  Use `--file-format` instead.")
    default_format = ".json"
    provided_file_format = None
    if args.format or args.file_format:
        provided_file_format = f".{(args.format or args.file_format).lower()}"
    file_is_stdout = _is_stdout(args.file)