tests: rename "kubectl" setupTestNamespace

As the other version has a different signature, but exists for a
different build tag. Resulting in my IDE becoming absolutely confused
when I tried to enable both at the same time. Opted for "exec" because
this one shells out.

Signed-off-by: Hidde Beydals <hello@hidde.co>

cmd/flux/source_oci_test.go

@@ -25,7 +25,7 @@ import (
 
 func TestSourceOCI(t *testing.T) {
 	namespace := allocateNamespace("oci-test")
-	del, err := setupTestNamespace(namespace)
+	del, err := execSetupTestNamespace(namespace)
 	if err != nil {
 		t.Fatal(err)
 	}