dashboard/app: fix parsing bug and accept "gs://" prefixes #14912

Job	Run time
build	10m 57s
aux	4m 49s
arch (presubmit_arch_linux)	10m 43s
dashboard	3m 13s
gvisor	4m 8s
arch (presubmit_arch_freebsd)	6m 30s
arch (presubmit_arch_netbsd)	4m 26s
old	6m 36s
arch (presubmit_arch_openbsd)	4m 25s
race	8m 56s
race_dashboard	4m 39s
arch (presubmit_arch_darwin)	4m 32s
arch (presubmit_arch_windows)	3m 22s
arch (presubmit_arch_executor)	3m 13s
	1h 20m 29s

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