Revert "internal/lsp/cache: disable GOPACKAGESDRIVER"

Now that https://github.com/bazelbuild/rules_go has a working prototype of a `GOPACKAGESDRIVER`, it may be time to revert that commit.

The draft implementation is at bazel-contrib/rules_go#2858.

Change-Id: Ia738e8be448d936f8a3b2b421d0a765f94bbff52
GitHub-Last-Rev: 0df6c91
GitHub-Pull-Request: golang#297
Reviewed-on: https://go-review.googlesource.com/c/tools/+/307169
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Trust: Rebecca Stambler <rstambler@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

internal/lsp/cache/snapshot.go

@@ -200,14 +200,10 @@ func (s *snapshot) config(ctx context.Context, inv *gocommand.Invocation) *packa
 	verboseOutput := s.view.options.VerboseOutput
 	s.view.optionsMu.Unlock()
 
-	// Forcibly disable GOPACKAGESDRIVER. It's incompatible with the
-	// packagesinternal APIs we use, and we really only support the go command
-	// anyway.
-	env := append(append([]string{}, inv.Env...), "GOPACKAGESDRIVER=off")
 	cfg := &packages.Config{
 		Context:    ctx,
 		Dir:        inv.WorkingDir,
-		Env:        env,
+		Env:        inv.Env,
 		BuildFlags: inv.BuildFlags,
 		Mode: packages.NeedName |
 			packages.NeedFiles |