fix: allow github action to work on external repos

[ethanneff]

👋🏻 Hi there!

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Thank you for helping us improve Image Actions! ⚡️

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What does this PR introduce?

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• adds AVIF support
• fixes #
• updates README.md with a better description for contributions

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• closes #

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[benschwarz]

Hey @ethanneff, thanks for the PR. Could you please sign the CLA and write a description of your changes and the motivations behind them?

At the moment the configuration you've changed is there purposefully to not optimize images on forked repositories. It would be good to hear why you think this behavior should be changed.