dashboard/config/linux: enable BB descriptors #19058

Job	Run time
race_dashboard	53s
race	1m 9s
old	3m 18s
aux	3m 18s
dashboard	1m 7s
gvisor	3m 17s
build	45s
arch (presubmit_arch_freebsd)	3m 18s
arch (presubmit_arch_darwin)	3m 20s
arch (presubmit_arch_netbsd)	3m 19s
arch (presubmit_arch_openbsd)	3m 19s
arch (presubmit_arch_linux)	3m 18s
arch (presubmit_arch_executor)	3m 16s
arch (presubmit_arch_windows)	3m 19s
	36m 56s

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