[ci] Don't use branch name for concurrency (#31296)

I happened to notice some jobs on main get canceled if another PR landed
before the prior commit on main had finished running CI. This is not
great for difftrain because the commit artifacts job relies on the CI
jobs on main finishing before it triggers. This would lead to commits
being skipped on DiffTrain which is not great for provenance since we
want it to be a 1:1 sync.

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[//]: # (BEGIN SAPLING FOOTER)
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with [ReviewStack](https://reviewstack.dev/facebook/react/pull/31296).
* #31297
* __->__ #31296