add xds/internal to avoid list of linter rule

scripts/vet.sh

@@ -51,7 +51,7 @@ git grep 'func (s) ' -- "*_test.go" | not grep -v 'func (s) Test'
 git grep 'func [A-Z]' -- "*_test.go" | not grep -v 'func Test\|Benchmark\|Example'
 
 # - Make sure all context usages are done timeout.
-git grep -e 'context.Background()' --or -e 'context.TODO()' -- "*_test.go" | grep -v "benchmark/primitives/context_test.go" | grep -v "credentials/google" | grep -v "internal/transport/" | not grep -v 'context.WithTimeout(' | not grep -v 'context.WithCancel('
+git grep -e 'context.Background()' --or -e 'context.TODO()' -- "*_test.go" | grep -v "benchmark/primitives/context_test.go" | grep -v "credentials/google" | grep -v "internal/transport/" | grep -v "xds/internal/" | not grep -v 'context.WithTimeout(' | not grep -v 'context.WithCancel('
 
 # - Do not use time.After except in tests.  It has the potential to leak the
 #   timer since there is no way to stop it early.