epyc: init hydra settings properly

hosts/epyc.nix

@@ -1,3 +1,8 @@
+{ lib, ... }:
+let
+  gcc-system-features = arch: lib.optionals (arch != null) ([ "gccarch-${arch}" ]
+    ++ map (x: "gccarch-${x}") lib.systems.architectures.inferiors.${arch});
+in
 {
   imports = [
     ../modules/ipmi-supermicro.nix
@@ -19,6 +24,17 @@
     };
   };
 
+  nix.buildMachines = [
+    { hostName = "localhost";
+      systems = [
+        "x86_64-linux"
+        "riscv64-linux"
+      ];
+      supportedFeatures = [ "kvm" "nixos-test" "big-parallel" "benchmark" ] ++ gcc-system-features "znver3";
+      maxJobs = 1;
+    }
+  ];
+
   boot.binfmt.emulatedSystems = [ "riscv64-linux" "aarch64-linux" "riscv64-linux" ];
 
   simd.arch = "znver3";