Bump pip from 25.3 to 26.0 in /kokoro/gcp_ubuntu #1816

Job	Run time
build (ubuntu-latest, 17, temurin)	22m 31s
build (ubuntu-latest, 26-ea, temurin)	16m 28s
build (ubuntu-latest, 21, temurin)	21m 35s
build (ubuntu-latest, 25, temurin)	16m 54s
	1h 17m 28s

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