switch back to LTS kernel due to Virtualbox build failure

nixos/configuration.nix

@@ -20,10 +20,7 @@ in
   services.davfs2.enable = true;
 
   # try newer kernels
-  boot.kernelPackages = pkgs.linuxPackages_latest;
-
-  # use old kernel because 5.4 and 5.5 might be buggy with intel graphics
-  #boot.kernelPackages = pkgs.linuxPackages_4_19;
+  #boot.kernelPackages = pkgs.linuxPackages_latest;
 
   services.fstrim.enable = true;
   services.btrfs.autoScrub =
@@ -85,7 +82,7 @@ in
   # Enable the OpenSSH daemon.
   # services.openssh.enable = true;
 
-  #programs.ssh.startAgent = true;
+  programs.ssh.startAgent = true;
 
   # Open ports in the firewall.
   # networking.firewall.allowedTCPPorts = [ ... ];
@@ -246,7 +243,7 @@ in
       keep-outputs = true;
       keep-derivations = true;
       trusted-users = [ "spiollinux" ];
-      experimental-features = [ "nix-command" "flakes" ];
+      experimental-features = [ "nix-command" "flakes" "repl-flake" ];
       # use all cores for building
       cores = 0;
     };