filter: skip the read-only directory test as root, ci's runner ignores mode bits

src/filter/filter_integration_test.zig

@@ -1067,6 +1067,8 @@ test "6: a 404 leaves the compiled files and the snapshot untouched" {
 
 test "6b: a local failure reports itself, not the previous source's network fault" {
     if (!build_options.integration) return error.SkipZigTest;
+    // Mode bits do not apply to root, so the denial the test needs cannot happen.
+    if (std.c.geteuid() == 0) return error.SkipZigTest;
 
     const gpa = testing.allocator;
     const env = try Env.create(gpa);