workflow: don't trigger gemini pr review by pull_request #17604

Job	Run time
build	3m 59s
race_dashboard	2m 49s
gvisor	3m 49s
race	8m 54s
aux	3m 56s
old	7m 6s
dashboard	2m 11s
arch (presubmit_arch_netbsd)	3m 44s
arch (presubmit_arch_freebsd)	6m 29s
arch (presubmit_arch_openbsd)	3m 40s
arch (presubmit_arch_darwin)	3m 45s
arch (presubmit_arch_windows)	3m 27s
arch (presubmit_arch_executor)	3m 12s
arch (presubmit_arch_linux)	10m 19s
	1h 7m 20s

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