Rollup merge of rust-lang#140470 - ojeda:rfl, r=lqd

CI: rfl: move job forward to Linux v6.15-rc4

A hopefully routine upgrade to Linux v6.15-rc4!

r? ```@lqd``` ```@Kobzol```
try-job: x86_64-rust-for-linux
```@rustbot``` label A-rust-for-linux
```@bors``` try

Author: Zalathar

src/ci/docker/scripts/rfl-build.sh

@@ -2,7 +2,7 @@
 
 set -euo pipefail
 
-LINUX_VERSION=v6.14-rc3
+LINUX_VERSION=v6.15-rc4
 
 # Build rustc, rustdoc, cargo, clippy-driver and rustfmt
 ../x.py build --stage 2 library rustdoc clippy rustfmt