Use cloned_from() instead of clone() in app.rs

[jfernandez]

To silence the clippy warning:

warning: assigning the result of `Clone::clone()` may be inefficient
   --> src/app.rs:279:9
    |
279 |         *self.graphs_bpf_program.lock().unwrap() = self.selected_program().clone();
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `clone_from()`: `self.graphs_bpf_program.lock().unwrap().clone_from(&self.selected_program())`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#assigning_clones
    = note: `#[warn(clippy::assigning_clones)]` on by default