[ci] Don't cancel runs if more than one branch triggers CI (#31848)

This might primarily only affect those using Sapling for React
development, but pushing the same commit to multiple branches shouldn't
cancel the run

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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/31848).
* __->__ #31848
* #31847
* #31846