go/build: set GO111MODULE=off explicitly in TestImportVendor*

These tests check for GOPATH-mode vendoring behavior, so make sure
they're in GOPATH mode.

Updates #30228

Change-Id: I646f59b67cb76dacd07adc3f6ed15ed63f4e22a4
Reviewed-on: https://go-review.googlesource.com/c/164620
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/go/build/build_test.go

@@ -350,6 +350,10 @@ func TestImportDirNotExist(t *testing.T) {
 
 func TestImportVendor(t *testing.T) {
 	testenv.MustHaveGoBuild(t) // really must just have source
+
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "off")
+
 	ctxt := Default
 	wd, err := os.Getwd()
 	if err != nil {
@@ -368,6 +372,10 @@ func TestImportVendor(t *testing.T) {
 
 func TestImportVendorFailure(t *testing.T) {
 	testenv.MustHaveGoBuild(t) // really must just have source
+
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "off")
+
 	ctxt := Default
 	wd, err := os.Getwd()
 	if err != nil {
@@ -387,6 +395,10 @@ func TestImportVendorFailure(t *testing.T) {
 
 func TestImportVendorParentFailure(t *testing.T) {
 	testenv.MustHaveGoBuild(t) // really must just have source
+
+	defer os.Setenv("GO111MODULE", os.Getenv("GO111MODULE"))
+	os.Setenv("GO111MODULE", "off")
+
 	ctxt := Default
 	wd, err := os.Getwd()
 	if err != nil {