gopls/internal/cache: ensure GO111MODULE is unset in GOPATH tests

With GO111MODULE=on in the test environment, the GOPATH subtest of
TestZeroConfigAlgorithm was failing, since GO111MODULE=on invalidated
our detection of a GOPATH workspace. Fix the test environment.

Fixes golang/go#70196

Change-Id: Ia80a5a63d93842aef4b67ca419e1d27eb11573c9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/625515
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Hongxiang Jiang <[email protected]>

Author: findleyr

gopls/internal/cache/session_test.go

@@ -85,7 +85,8 @@ func TestZeroConfigAlgorithm(t *testing.T) {
 				options: func(dir string) map[string]any {
 					return map[string]any{
 						"env": map[string]any{
-							"GOPATH": dir,
+							"GO111MODULE": "", // golang/go#70196: must be unset
+							"GOPATH":      dir,
 						},
 					}
 				},