Revert "linux-hardened: 5.7 -> 5.8"

This reverts commit 4712b94.
This broke a test and has been blocking the channel for over a week.
(see 4712b946e )

pkgs/top-level/all-packages.nix

@@ -17933,7 +17933,7 @@ in
   # Hardened Linux
   hardenedLinuxPackagesFor = kernel': overrides:
     let # Note: We use this hack since the hardened patches can lag behind and we don't want to delay updates:
-      linux_latest_for_hardened = pkgs.linux_5_8;
+      linux_latest_for_hardened = pkgs.linux_5_7;
       kernel = (if kernel' == pkgs.linux_latest then linux_latest_for_hardened else kernel').override overrides;
     in linuxPackagesFor (kernel.override {
       structuredExtraConfig = import ../os-specific/linux/kernel/hardened/config.nix {