Run uploads in the background, to speed builds

modules/nixos/remote-builder.nix

@@ -40,17 +40,30 @@ with lib;
     greg.tailscale.enable = true;
 
     # The builder user needs to be trusted to submit builds
-    nix.settings = {
+    nix.settings =
-      post-build-hook = getExe (
+      let
+        upload = getExe (
           pkgs.writeShellScriptBin "upload-to-cache.sh" ''
             set -eu
             set -f
             export AWS_SHARED_CREDENTIALS_FILE=${config.age.secrets.cache-credentials.path}
             export IFS=' '
-          echo "Uploading paths " $OUT_PATHS
+            ${getExe config.nix.package} copy --to "s3://binary-cache/?scheme=http&endpoint=nas.home%3A9000&profile=default" "$@"
-          nix copy --to "s3://binary-cache/?scheme=http&endpoint=nas.home%3A9000&profile=default" $OUT_PATHS
           ''
         );
+        uploadRunner = getExe (
+          pkgs.writeShellScriptBin "uploade-to-cache-runner.sh" ''
+            ${lib.getExe' config.systemd.package "systemd-run"} \
+                --unit "upload-$(${lib.getExe' pkgs.coreutils "date"} +%s%3N)" \
+                --property Type=exec \
+                --property CollectMode=inactive \
+                --property Group=nixbld \
+                ${upload} $OUT_PATHS
+          ''
+        );
+      in
+      {
+        post-build-hook = uploadRunner;
         secret-key-files = config.age.secrets.private-cache.path;
         trusted-users = [ config.users.users.remote-builder-user.name ];
       };