Use nightly Clippy

rust-lang/rust-clippy#10264 was fixed by
rust-lang/rust-clippy#10391.

Use nightly Clippy to get the fix.

core/src/backup.rs

@@ -2,10 +2,6 @@
 // https://github.com/smoelius/rustfmt_if_chain/blob/557c32c54b0e0f48da2d029a3a8f70db4c8dbf9b/src/backup.rs
 
 #![allow(dead_code)]
-// smoelius: Allow `unwrap_used` until the following issue is resolved:
-// https://github.com/rust-lang/rust-clippy/issues/10264
-#![allow(clippy::unwrap_used)]
-#![cfg_attr(dylint_lib = "overscoped_allow", allow(overscoped_allow))]
 
 use std::{
     io::Result,

core/tests/ci_is/enabled.rs

@@ -175,7 +175,7 @@ fn update() {
 fn clippy_command(cargo_args: &[&str], rustc_args: &[&str]) -> Command {
     let mut command = Command::new("cargo");
     command
-        .args(["clippy", "--all-features", "--all-targets"])
+        .args(["clippy", "+nightly", "--all-features", "--all-targets"])
         .args(cargo_args)
         .args(["--"])
         .args(rustc_args)