chore(nixbuild): add supportedFeatures to builder configs

- Introduced `supportedFeatures` to each builder for better control over build capabilities (`nixos-test`, `benchmark`, `big-parallel`, and `kvm` for SGX).
- Enabled `builders-use-substitutes` setting to optimize build efficiency.

systems/nixbuild.nix

@@ -10,6 +10,11 @@
       speedFactor = 3;
       sshUser = "harald";
       sshKey = "/etc/ssh/nix-builder-key";
+      supportedFeatures = [
+        "nixos-test"
+        "benchmark"
+        "big-parallel"
+      ];
     }
     {
       hostName = "rialo";
@@ -18,6 +23,11 @@
       speedFactor = 3;
       sshUser = "harald";
       sshKey = "/etc/ssh/nix-builder-key";
+      supportedFeatures = [
+        "nixos-test"
+        "benchmark"
+        "big-parallel"
+      ];
     }
     {
       hostName = "sgx";
@@ -26,9 +36,17 @@
       speedFactor = 1;
       sshUser = "harald";
       sshKey = "/etc/ssh/nix-builder-key";
+      supportedFeatures = [
+        "nixos-test"
+        "benchmark"
+        "big-parallel"
+        "kvm"
+      ];
     }
   ];
 
+  nix.settings.builders-use-substitutes = true;
+
   # Ensure Nix is in PATH for SSH sessions
   programs.fish.shellInit = ''
     set -gx PATH /nix/var/nix/profiles/default/bin $PATH