workflow: don't trigger gemini pr review by pull_request #17602

Job	Run time
race	7m 16s
build	4m 11s
old	7m 6s
aux	3m 57s
race_dashboard	3m 29s
dashboard	2m 30s
arch (presubmit_arch_linux)	10m 20s
arch (presubmit_arch_openbsd)	3m 39s
arch (presubmit_arch_netbsd)	3m 51s
gvisor	3m 45s
arch (presubmit_arch_freebsd)	6m 20s
arch (presubmit_arch_windows)	3m 38s
arch (presubmit_arch_darwin)	3m 42s
arch (presubmit_arch_executor)	3m 20s
	1h 7m 4s

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