Rollup merge of #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: matthiaskrgr