docs(gatsby-plugin-preload-fonts): add external chrome env var

[nibtime]

Description

A hint, that Puppeteer can be configured with environment variables, which is required in some configurations (e.g. Alpine Linux)

Related Issues

fixes #21778

[wardpeet]

Thanks! Great addition to the Readme of this plugin. Thank you! 🙏

[gatsbot]

Holy buckets, @nibtime — we just merged your PR to Gatsby! 💪💜

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