Allow overriding the command passed to Expo

[lawrencejones]

You may want to call into the command action using a provided command string rather than what comes from the GitHub Action.

This is useful if you want to write an action that listens for a specific shortcut (e.g. #release production) which then calls the expo/eas command supported by the official action. Then you benefit from the nice shortcut, remove the possibility of misconfiguration, while keeping the official GHA's behaviour like posting builds back to the PR or responding to the original comment.

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