tools/check-commits.sh: allow AUTHORS as a prefix in commit message summaries #18424

Job	Run time
old	7m 17s
dashboard	2m 13s
build	4m 24s
race_dashboard	3m 17s
aux	4m 42s
race	8m 53s
gvisor	3m 56s
arch (presubmit_arch_darwin)	3m 44s
arch (presubmit_arch_openbsd)	3m 46s
arch (presubmit_arch_netbsd)	3m 47s
arch (presubmit_arch_executor)	3m 16s
arch (presubmit_arch_linux)	10m 41s
arch (presubmit_arch_windows)	3m 34s
arch (presubmit_arch_freebsd)	6m 26s
	1h 9m 56s

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