fix: Prevent getCompletions from modifying os.Args

[jonathan-conder]

Possible fix for #2257.

Tested using this test case:

func TestModifiedArgs(t *testing.T) {
	cmd := &Command{TraverseChildren: true}
	args := []string{"__complete", ""}
	cmd.SetArgs(args)
	if err := cmd.Execute(); err != nil {
		t.Errorf("Unexpected error: %v", err)
	}
	if len(args) != 2 || args[1] != "" {
		t.Error("Modified args")
	}
}

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