fix(remote-builders): serve x86_64 only, drop aquamarine binfmt (#517)

Assisted-by: DeepSeek Harness (deepseek-flash)

outputs/aarch64-linux/tests/remote-builders/expected.nix

@@ -1,4 +1,5 @@
 {
   distributedBuilds = true;
   aquamarineConfigured = true;
+  builderSystems = [ [ "x86_64-linux" ] ];
 }

outputs/aarch64-linux/tests/remote-builders/expr.nix

@@ -11,4 +11,6 @@ in
   aquamarineConfigured = builtins.any (
     machine: machine.hostName == "aquamarine" && machine.sshUser == "root"
   ) cfg.nix.buildMachines;
+  # shoukei builds aarch64 natively, so the builder must not claim it.
+  builderSystems = map (machine: machine.systems) cfg.nix.buildMachines;
 }

outputs/x86_64-linux/tests/remote-builders/expected.nix

@@ -13,4 +13,10 @@ in
 {
   distributedBuilds = yes;
   aquamarineConfigured = yes;
+  builderSystems = lib.genAttrs [
+    "ai-niri"
+    "shoryu"
+    "shushou"
+    "youko"
+  ] (_: [ [ "x86_64-linux" ] ]);
 }

outputs/x86_64-linux/tests/remote-builders/expr.nix

@@ -19,4 +19,9 @@ in
 {
   distributedBuilds = lib.genAttrs builderHosts (name: (cfg name).nix.distributedBuilds);
   aquamarineConfigured = lib.genAttrs builderHosts usesAquamarine;
+  # The builder must advertise only its native platform: an aarch64 build there
+  # runs under qemu-user and loses to building it on the client.
+  builderSystems = lib.genAttrs builderHosts (
+    name: map (machine: machine.systems) (cfg name).nix.buildMachines
+  );
 }