dashboard/app: fix up pprof handlers access #18476

Job	Run time
race_dashboard	3m 26s
build	4m 33s
old	7m 1s
aux	3m 58s
arch (presubmit_arch_netbsd)	3m 49s
arch (presubmit_arch_windows)	3m 32s
dashboard	2m 23s
gvisor	4m 4s
arch (presubmit_arch_freebsd)	6m 32s
race	7m 14s
arch (presubmit_arch_linux)	10m 51s
arch (presubmit_arch_executor)	3m 19s
arch (presubmit_arch_darwin)	3m 40s
arch (presubmit_arch_openbsd)	3m 47s
	1h 8m 9s

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