Auto merge of #29793 - servo:layout-2020-nightly-builds, r=atbrakhi

Upload nightly builds for layout2020 variant

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We'd like to start offering servo nightly builds with layout 2020 engine so that users can test the new layout without building servo. These new builds will be offered in addition to the current 2013 builds until we switch over to 2020 as the default.

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

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- [ ] There are tests for these changes OR
- [x] These changes do not require tests because they extend nightly CI job.

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