pkg/cover: reduce generateReport cyclo #15790

Job	Run time
aux	3m 50s
dashboard	2m 48s
build	4m 16s
arch (presubmit_arch_linux)	10m 19s
gvisor	3m 31s
old	6m 48s
arch (presubmit_arch_freebsd)	6m 7s
arch (presubmit_arch_netbsd)	3m 43s
race	9m 47s
arch (presubmit_arch_openbsd)	3m 37s
race_dashboard	3m 54s
arch (presubmit_arch_darwin)	3m 41s
arch (presubmit_arch_windows)	3m 29s
arch (presubmit_arch_executor)	3m 23s
	1h 9m 13s

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