dashboard/app: fix create_upload_url bug #14886

Job	Run time
dashboard	3m 16s
build	11m 45s
arch (presubmit_arch_linux)	10m 50s
aux	4m 54s
arch (presubmit_arch_freebsd)	6m 37s
arch (presubmit_arch_netbsd)	4m 27s
gvisor	4m 12s
arch (presubmit_arch_openbsd)	4m 21s
old	6m 39s
arch (presubmit_arch_darwin)	4m 22s
race	8m 43s
arch (presubmit_arch_windows)	3m 23s
race_dashboard	4m 22s
arch (presubmit_arch_executor)	3m 21s
	1h 21m 12s

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