Ignore buggy clippy::lint_groups_priority lint

```
error: lint group `rust_2018_idioms` has the same priority (0) as a lint
  --> Cargo.toml:28:1
   |
28 | rust_2018_idioms = "warn"
   | ^^^^^^^^^^^^^^^^   ------ has an implicit priority of 0
29 | single_use_lifetimes = "warn"
   | -------------------- has the same priority as this lint
   |
   = note: the order of the lints in the table is ignored by Cargo
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#lint_groups_priority
   = note: `#[deny(clippy::lint_groups_priority)]` on by default
help: to have lints override the group set `rust_2018_idioms` to a lower priority
   |
28 | rust_2018_idioms = { level = "warn", priority = -1 }
   |                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Cargo.toml

@@ -28,3 +28,5 @@ tokio = "1"
 rust_2018_idioms = "warn"
 single_use_lifetimes = "warn"
 unreachable_pub = "warn"
+[workspace.lints.clippy]
+lint_groups_priority = { level = "allow", priority = 1 } # https://github.com/rust-lang/rust-clippy/issues/12920