cmd/go: quote entries in list-valued variables for go env in plan9
When 'go env' without an argument prints environment variables as
a script which can be executed by the shell, variables with a
list value in Plan 9 (such as GOPATH) need to be printed with each
element enclosed in single quotes in case it contains characters
significant to the Plan 9 shell (such as ' ' or '=').
For #58508
Change-Id: Ia30f51307cc6d07a7e3ada6bf9d60bf9951982ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/493535
Run-TryBot: Cherry Mui <cherryyz@google.com> Reviewed-by: Cherry Mui <cherryyz@google.com> Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>