go/mod: update, add toolchain #15469

Job	Run time
aux	4m 40s
build	1m 45s
dashboard	2m 56s
arch (presubmit_arch_linux)	9m 31s
gvisor	3m 23s
old	6m 22s
arch (presubmit_arch_freebsd)	5m 54s
race	9m 52s
arch (presubmit_arch_netbsd)	3m 59s
race_dashboard	3m 42s
arch (presubmit_arch_openbsd)	3m 47s
arch (presubmit_arch_darwin)	3m 31s
arch (presubmit_arch_windows)	3m 20s
arch (presubmit_arch_executor)	3m 14s
	1h 5m 56s

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