linux_testing: fix Rust support with current rustc

This won't build at present, because a fix for the rustc wrapper needs
to go through staging.

pkgs/top-level/linux-kernels.nix

···
         kernelPatches = [
           kernelPatches.bridge_stp_helper
           kernelPatches.request_key_helper
+          kernelPatches.rust_1_75
         ];
       };
       latest = packageAliases.linux_latest.kernel;