Improve efficiency of jobs mutex in prism server and prevent race conditions in artifacts map.

[jeremyje]

Resolves #32656

This change adds a mutex to the artifacts variable in the grpc server for the jobservices in the Prism runner within the Go SDK. In general, maps in Go are not thread safe so 2 concurrent grpc requests that read/write from any of the handlers, notably /ArtifactRetrievalService.ResolveArtifacts /ArtifactStagingService.ReverseArtifactRetrievalService handlers will trigger a race condition. This generally is not visible unless you have a test with the race detector enabled.

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