nix: Allow all capabilities in Nomad

This will be necessary for the smartctl exporter since it needs Linux
capabilities that are not allowed by default in Nomad.

We only have trusted Nomad jobs, and we already allow privileged
containers anyway, so there is no security impact.

nix/deuxfleurs.nix

@@ -397,6 +397,7 @@ in
                 {
                   volumes.enabled = true;
                   allow_privileged = true;
+                  allow_caps = ["all"];
                 }
               ];
             }