build: run samples with Emulator version 1.5.42

There seems to be a regression in Emulator version 1.5.43 with regards to
pending_commit_timestamp() that causes at least one sample to fail.

Author: olavloite

samples/sample_helper.py

@@ -53,7 +53,7 @@ def run_sample(sample_method: Callable):
 
 def start_emulator() -> (DockerContainer, str):
     emulator = DockerContainer(
-        "gcr.io/cloud-spanner-emulator/emulator"
+        "gcr.io/cloud-spanner-emulator/emulator:1.5.42"
     ).with_exposed_ports(9010)
     emulator.start()
     wait_for_logs(emulator, "gRPC server listening at 0.0.0.0:9010")