internal/lsp/cache: let Session.getGoEnv query GOWORK

This will make GOWORK value appear in the gopls log
when initializing the workspace. This CL does not include
GOWORK in `environmentVariables` (in view.go) (yet).

Change-Id: I16224d1ac50ac8209def008966371c6be858d05a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/385894
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>

internal/lsp/cache/view.go

@@ -936,6 +936,7 @@ func (s *Session) getGoEnv(ctx context.Context, folder string, goversion int, go
 	for k := range vars {
 		args = append(args, k)
 	}
+	args = append(args, "GOWORK")
 
 	inv := gocommand.Invocation{
 		Verb:       "env",