### 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)

ARG_ONLY_RUNNING = Arg(("-r", "--only-running"), help="Only running jobs", action="store_true")
ARG_DOWNSTREAM = Arg(("-d", "--downstream"), help="Include downstream tasks", action="store_true")
ARG_EXCLUDE_SUBDAGS = Arg(("-x", "--exclude-subdags"), help="Exclude subdags", action="store_true")
ARG_EXCLUDE_PARENTDAG = Arg(
    ("-X", "--exclude-parentdag"),
    help="Exclude ParentDAGS if the task cleared is a part of a SubDAG",
    action="store_true",
)
ARG_DAG_REGEX = Arg(
    ("-R", "--dag-regex"), help="Search dag_id as regex instead of exact string", action="store_true"
)
ARG_SAVE = Arg(("-s", "--save"), help="Saves the result to the indicated file.")
ARG_IMGCAT = Arg(("--imgcat",), help="Displays graph using the imgcat tool.", action="store_true")
ARG_RUN_ID = Arg(("-r", "--run-id"), help="Helps to identify this run")
ARG_CONF = Arg(("-c", "--conf"), help="JSON string that gets pickled into the DagRun's conf attribute")
ARG_EXEC_DATE = Arg(("-e", "--exec-date"), help="The execution date of the DAG", type=parsedate)
ARG_REPLACE_MICRO = Arg(
    ("--no-replace-microseconds",),
    help="whether microseconds should be zeroed",
    dest="replace_microseconds",
    action="store_false",
    default=True,
)
ARG_DB_TABLES = Arg(
    ("-t", "--tables"),
    help=lazy_object_proxy.Proxy(
        lambda: f"Table names to perform maintenance on (use comma-separated list).\n"
        f"Options: {import_string('airflow.cli.commands.db_command.all_tables')}"
    ),
    type=string_list_type,