fix(users/flokli/nixos-tvix-cache): set timeInterval for metrics DS

The data source defaults to 15s of time interval. As alloy scrapes every
60s only, this causes watching dashboards with a smaller time range to
just not show any data, like the CPU graph being empty for a time range
< last 12h.

Fix by setting time interval to 60s.

Co-Authored-By: WilliButz <willibutz@posteo.de>
Change-Id: Ife306b2fda968654cad818a82f99e0011819be3c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12923
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>

users/flokli/nixos/nixos-tvix-cache/monitoring.nix

@@ -165,6 +165,9 @@ in
           type = "prometheus";
           uid = "mimir";
           url = "http://localhost:9009/prometheus";
+          jsonData = {
+            timeInterval = "60s";
+          };
         }
       ];
     };