Bump virtualenv from 20.26.6 to 20.36.1 in /kokoro/gcp_ubuntu #1772

Job	Run time
build (ubuntu-latest, 17, temurin)	22m 19s
build (ubuntu-latest, 21, temurin)	21m 50s
build (ubuntu-latest, 25, temurin)	17m 6s
build (ubuntu-latest, 26-ea, temurin)	17m 16s
	1h 18m 31s

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