x/build: coordinator shouldn't wait forever for builders that do not snapshot

[toothrot]

In https://golang.org/cl/361754 (#49207), a builder was corrected to SkipSnapshot when it doesn't build the go repo. The following code will loop forever in this scenario, as a snapshot will never be uploaded for this configuration:

https://cs.opensource.google/go/x/build/+/master:cmd/coordinator/coordinator.go;l=1016;drc=0f6a98d88fcd2f596743aecbdc5b3def2458eded

A test should fail if this invalid configuration occurs.