pkg/osutil: add a WriteJSON helper #14924

Job	Run time
dashboard	3m 41s
build	11m 32s
aux	4m 50s
arch (presubmit_arch_linux)	10m 51s
old	6m 38s
race	8m 47s
arch (presubmit_arch_freebsd)	6m 35s
arch (presubmit_arch_netbsd)	4m 24s
race_dashboard	4m 26s
arch (presubmit_arch_openbsd)	4m 19s
gvisor	4m 15s
arch (presubmit_arch_darwin)	4m 17s
arch (presubmit_arch_windows)	3m 22s
arch (presubmit_arch_executor)	3m 22s
	1h 21m 19s

Provide feedback