chore(spanner): run it for mux rw #10603

Job	Run time
units (11)	3m 0s
units (17)	2m 44s
units (21)	2m 43s
units (8)	2m 54s
units-with-multiplexed-session (8)	3m 23s
units-with-multiplexed-session (11)	3m 19s
units-with-multiplexed-session (17)	3m 5s
units-with-multiplexed-session (21)	3m 9s
clirr	1m 11s
dependencies (17)	1m 12s
javadoc	2m 34s
lint	1m 14s
windows	18m 58s
	49m 26s

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