dashboard/app: provide a text filename to fsck logs #14883

Job	Run time
dashboard	3m 36s
aux	4m 57s
build	10m 53s
arch (presubmit_arch_linux)	11m 1s
race	8m 39s
gvisor	4m 22s
arch (presubmit_arch_freebsd)	6m 29s
arch (presubmit_arch_netbsd)	4m 23s
race_dashboard	4m 3s
arch (presubmit_arch_openbsd)	4m 16s
old	6m 34s
arch (presubmit_arch_darwin)	4m 15s
arch (presubmit_arch_windows)	3m 20s
arch (presubmit_arch_executor)	3m 22s
	1h 20m 10s

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