dashboard/app: add manager param to /cover?jsonl=1 output #16693

Job	Run time
aux	5m 19s
build	3m 23s
dashboard	2m 11s
race	7m 13s
arch (presubmit_arch_linux)	10m 20s
arch (presubmit_arch_freebsd)	6m 9s
arch (presubmit_arch_openbsd)	3m 45s
arch (presubmit_arch_netbsd)	3m 52s
arch (presubmit_arch_windows)	3m 47s
arch (presubmit_arch_darwin)	3m 59s
arch (presubmit_arch_executor)	3m 22s
gvisor	4m 4s
race_dashboard	3m 41s
old	6m 47s
	1h 7m 52s

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