Replace erroneous PUPPETEER_SKIP_DOWNLOAD

[make-github-pseudonymous-again]

PUPPETEER_SKIP_DOWNLOAD ~ PUPPETEER_SKIP_CHROMIUM_DOWNLOAD.

[google-cla]

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[make-github-pseudonymous-again]

I will not sign an agreement for a typo edit. Feel free to close this PR and fix in another commit.

[mathiasbynens]

Good catch!