Rollup merge of #138331 - nnethercote:use-RUSTC_LINT_FLAGS-more, r=onur-ozkan,jieyouxu

Use `RUSTC_LINT_FLAGS` more

An alternative to the failed #138084.

Fixes #138106.

r? ````@jieyouxu````

compiler/rustc_parse_format/src/lib.rs

@@ -13,7 +13,6 @@
     html_playground_url = "https://play.rust-lang.org/",
     test(attr(deny(warnings)))
 )]
-#![warn(unreachable_pub)]
 // tidy-alphabetical-end
 
 pub use Alignment::*;