fix(build): ignore GITHUB_REF in mdn/yari

[caugner]

Summary

Problem

On stage, the "View this page on GitHub" links to the mdn/content repo always point to the next branch, even though the build used the main content branch, because the build system uses the GITHUB_REF environment variable.

Solution

Ignore the GITHUB_REF environment variable in mdn/yari.

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How did you test this change?

Deployed to stage and tested that "View this page on GitHub" on https://developer.allizom.org/en-US/docs/Learn/HTML links to the right content branch (main).