Run uploads in the background, to speed builds

modules/nixos/remote-builder.nix

@@ -40,20 +40,33 @@ with lib;
     greg.tailscale.enable = true;
 
     # The builder user needs to be trusted to submit builds
-    nix.settings = {
-      post-build-hook = 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
-          nix copy --to "s3://binary-cache/?scheme=http&endpoint=nas.home%3A9000&profile=default" $OUT_PATHS
-        ''
-      );
-      secret-key-files = config.age.secrets.private-cache.path;
-      trusted-users = [ config.users.users.remote-builder-user.name ];
-    };
+    nix.settings =
+      let
+        upload = getExe (
+          pkgs.writeShellScriptBin "upload-to-cache.sh" ''
+            set -eu
+            set -f
+            export AWS_SHARED_CREDENTIALS_FILE=${config.age.secrets.cache-credentials.path}
+            export IFS=' '
+            ${getExe config.nix.package} copy --to "s3://binary-cache/?scheme=http&endpoint=nas.home%3A9000&profile=default" "$@"
+          ''
+        );
+        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 ];
+      };
 
     users.users.remote-builder-user = {
       openssh.authorizedKeys.keys = [