apidiff: reduce file permissions in test

Change-Id: I274cb27166bc5f86c8e977a381aef871d8987056
Reviewed-on: https://go-review.googlesource.com/c/exp/+/445577
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
diff --git a/apidiff/apidiff_test.go b/apidiff/apidiff_test.go
index 9f15595..40cd463 100644
--- a/apidiff/apidiff_test.go
+++ b/apidiff/apidiff_test.go
@@ -62,7 +62,7 @@
 	if err := os.MkdirAll(filepath.Join(dir, "src", "apidiff"), 0700); err != nil {
 		t.Fatal(err)
 	}
-	if err := os.WriteFile(filepath.Join(dir, "src", "apidiff", "go.mod"), []byte("module apidiff\ngo 1.18\n"), 0666); err != nil {
+	if err := os.WriteFile(filepath.Join(dir, "src", "apidiff", "go.mod"), []byte("module apidiff\ngo 1.18\n"), 0600); err != nil {
 		t.Fatal(err)
 	}