go/tools/builders: don't format subcommand output as []byte (#2766)
go/tools/builders: don't format subcommand output as []byte (#2766)

Use os.Stderr.Write instead of fmt.Fprint. The latter formats the
value as a []byte, not a string.

(This was due to my own bad review comment on #2736. Sorry.)
bazelbuild/rules_go,bazelbuild/rules_go,bazelbuild/rules_go,bazelbuild/rules_go,bazelbuild/rules_go