Enable 'copyloopvar' linter

internal/httputil/securityheader/securityheader_test.go

@@ -1,4 +1,4 @@
-// Copyright 2020-2022 the Pinniped contributors. All Rights Reserved.
+// Copyright 2020-2024 the Pinniped contributors. All Rights Reserved.
 // SPDX-License-Identifier: Apache-2.0
 
 package securityheader
@@ -56,7 +56,6 @@ func TestWrap(t *testing.T) {
 			},
 		},
 	} {
-		tt := tt
 		t.Run(tt.name, func(t *testing.T) {
 			testServer := httptest.NewServer(tt.wrapFunc(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 				w.Header().Set("X-Test-Header", "test value")