Fix permissions_set_readonly_false warning

[dtolnay]

warning: call to `set_readonly` with argument `false`
  --> src/cargo.rs:83:17
   |
83 |                 permissions.set_readonly(false);
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: on Unix platforms this results in the file being world writable
   = help: you can set the desired permissions using `PermissionsExt`. For more information, see
           https://doc.rust-lang.org/std/os/unix/fs/trait.PermissionsExt.html
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#permissions_set_readonly_false
   = note: `-W clippy::permissions-set-readonly-false` implied by `-W clippy::all`
   = help: to override `-W clippy::all` add `#[allow(clippy::permissions_set_readonly_false)]`