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Resolves #5600

This ignores the --webbrowser-headless config in the Twitch plugin and always forces headless to be False, because Twitch is able to detect Chromium's headless mode when forcing the acquirement of the client-integrity token. You can check this easily by setting an invalid OAuth token for example (--twitch-api-header="Authorization=OAuth invalid") or by modifying the plugin.

The CI tokens are currently not a requirement, but this can change at any time, and if they do and Streamlink has headless mode enabled by default (which is the case), then users are forced to update their configs or CLI arguments until we publish a new release, which is unnecessary. So let's set headless to False in the Twitch plugin now.