efi/preinstall: RunChecksContext.Run should return immediately if there is a pending PPI operation

After executing a PPI operation, there needs to be either a reboot or shutdown for the operation to be handled by the platform firmware, as indicated by the result of the call to `RunChecksContext.Run`. In this case, subsequent calls to `RunChecksContext.Run` should return immediately with the same response as opposed to re-running tests, as the operation may clear more than one error.

To implement this properly, we need to be able to obtain the pending PPI operation via the go-tpm2 `ppi.PPI` interface, but it isn't supported yet.