hydra: fix platforms, try 2

hosts/hydra/hydra.nix

@@ -61,7 +61,6 @@
       allowed-uris = http:// https:// ssh://
       builders-use-substitutes = true
       experimental-features = ca-derivations nix-command flakes
-      extra-platforms = riscv64-linux
     '';
     trustedUsers = [ "hydra" "root" ];
     buildMachines = [{
@@ -81,6 +80,10 @@
   services = {
     hydra-dev = {
       enable = true;
+      buildMachinesFiles = [
+        "/etc/nix/machines"
+        "/var/lib/hydra/machines"
+      ];
       hydraURL = "https://hydra.hq.c3d2.de";
       logo = ./c3d2.svg;
       minimumDiskFree = 1;
@@ -138,6 +141,16 @@
       MemorySwapMax = "64G";
     };
 
+    hydra-init.preStart = let
+      platforms = [ "x86_64-linux" ]
+        # use dacbert for arm
+        ++ lib.filter (e: e != "aarch64-linux" && !(lib.hasPrefix "armv" e)) config.nix.settings.extra-platforms;
+    in ''
+      cat << EOF > ~/machines
+      localhost ${lib.concatStringsSep "," platforms} - 4 5 - -
+      EOF
+    '';
+
     nix-daemon.serviceConfig = {
       CPUWeight = 5;
       MemoryHigh = "64G";