[ci] Use correct revision for Meta builds (#32680)

There was a bug previously in our commit artifacts step where the
emitted REVISION hash would reference the commit on the builds branch
rather than from `main`.

Given that our internal manual sync script also does this, let's align
them both to always reference the commit from `main` instead.
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[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/32680).
* __->__ #32680
* #32679
* #32678