[7.3.0] Deduplicate locally executed path mapped spawns (#23069)

When path mapping is enabled, different `Spawn`s in the same build can
have identical `RemoteAction.ActionKey`s and can thus provide remote
cache hits for each other. However, cache hits are only possible after
the first local execution has concluded and uploaded its result to the
cache.

To avoid unnecessary duplication of local work, the first `Spawn` for
each `RemoteAction.ActionKey` is tracked until its results have been
uploaded and all other concurrently scheduled `Spawn`s wait for it and
then copy over its local outputs.

Fixes #21043

Closes #22556.

PiperOrigin-RevId: 655097996
Change-Id: I4368f9210c67a306775164d252aae122d8b46f9b

Closes #23060