dashboard/app: fix create_upload_url bug #14884

Job	Run time
aux	4m 55s
dashboard	3m 23s
build	11m 28s
arch (presubmit_arch_linux)	11m 1s
gvisor	4m 23s
arch (presubmit_arch_freebsd)	6m 32s
old	6m 39s
race	8m 40s
arch (presubmit_arch_netbsd)	4m 27s
arch (presubmit_arch_openbsd)	4m 16s
race_dashboard	5m 11s
arch (presubmit_arch_darwin)	4m 20s
arch (presubmit_arch_windows)	3m 26s
arch (presubmit_arch_executor)	3m 16s
	1h 21m 57s

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