internal/lsp/fake: explicitly set GOPACKAGESDRIVER=off in regtests

In the past, changes to GOPACKAGESDRIVER have led to some confusing
regtest failures. Explicitly set it off.

Updates golang/go#39384

Change-Id: I303a58380a5e46e6621c19b2edc40d43199bb343
Reviewed-on: https://go-review.googlesource.com/c/tools/+/240058
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

internal/lsp/fake/sandbox.go

@@ -107,6 +107,7 @@ func (sb *Sandbox) GoEnv() []string {
 		"GOPROXY=" + sb.Proxy.GOPROXY(),
 		"GO111MODULE=",
 		"GOSUMDB=off",
+		"GOPACKAGESDRIVER=off",
 	}
 	if testenv.Go1Point() >= 5 {
 		vars = append(vars, "GOMODCACHE=")