linux_testing: 6.4-rc4 -> 6.4-rc6

https://lwn.net/ml/linux-kernel/CAHk-%3DwifuPqAFXQQTTLkp_%2BFMzxGFHpSG-hEtZazG-46s%3DnoAw%40mail.gmail.com/
https://lwn.net/ml/linux-kernel/CAHk-%3DwgmbjQu3bVSa5JJhEymeQhkr7N3cjg-CoNLVituUnfW8w%40mail.gmail.com/

pkgs/os-specific/linux/kernel/linux-testing.nix

@@ -3,7 +3,7 @@
 with lib;
 
 buildLinux (args // rec {
-  version = "6.4-rc4";
+  version = "6.4-rc6";
   extraMeta.branch = lib.versions.majorMinor version;
 
   # modDirVersion needs to be x.y.z, will always add .0
@@ -11,7 +11,7 @@ buildLinux (args // rec {
 
   src = fetchzip {
     url = "https://git.kernel.org/torvalds/t/linux-${version}.tar.gz";
-    hash = "sha256-PlxGRb4wKjamEDrSWpKXLxa7aX9lQoDgrjjrWhArisk=";
+    hash = "sha256-gJSVjuYoA5k7XuxRirS/ac770ZfXqIUvI7BUPwxvN1g=";
   };
 
   # Should the testing kernels ever be built on Hydra?