alloy: Standardize unix exporter job label to node

modules/nixos/services/alloy.nix

@@ -80,9 +80,17 @@ in
               enable_collectors = ["systemd", "processes"]
             }
 
-            prometheus.scrape "node_exporter" {
-              job_name        = "node"
+            discovery.relabel "unix_exporter" {
               targets = prometheus.exporter.unix.default.targets
+
+              rule {
+                target_label = "job"
+                replacement = "node"
+              }
+            }
+
+            prometheus.scrape "node_exporter" {
+              targets = discovery.relabel.unix_exporter.output
               forward_to = [prometheus.remote_write.default.receiver]
               scrape_interval = "15s"
             }