Monitoring: Fix backup duration alert

This should implement my actual intentions: Alert when
backups run for longer than 3h, and the the repo check
for more than 6h.

nixos/modules/monitoring/server/grafana-dashboards/backups.json

@@ -638,8 +638,8 @@
             "query": {
               "params": [
                 "A",
-                "3h",
-                "now"
+                "5m",
+                "now-3h"
               ]
             },
             "reducer": {
@@ -772,8 +772,8 @@
             "query": {
               "params": [
                 "A",
-                "6h",
-                "now"
+                "5m",
+                "now-6h"
               ]
             },
             "reducer": {