cmd/go: include GOMODCACHE in 'go help environment'

Updates #34527
Fixes #40089

Change-Id: Ie9c8573536e5c31e874d755f4d888ffc805b796f
Reviewed-on: https://go-review.googlesource.com/c/go/+/241275
Run-TryBot: Jay Conrod <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: Jay Conrod

src/cmd/go/alldocs.go

@@ -493,6 +493,8 @@ General-purpose environment variables:
 	GOCACHE
 		The directory where the go command will store cached
 		information for reuse in future builds.
+	GOMODCACHE
+		The directory where the go command will store downloaded modules.
 	GODEBUG
 		Enable various debugging facilities. See 'go doc runtime'
 		for details.