Use hut-substituter module in owl{1,2} and raccoon

`nix-serve` does not handle duplicates in the path:
```
hut$ curl http://127.0.0.1:5000/nix-cache-info
StoreDir: /nix/store
WantMassQuery: 1
Priority: 30
hut$ curl http://127.0.0.1:5000//nix-cache-info
File not found.
```

This meant that the cache was not accessible via:
`curl https://jungle.bsc.es/cache/nix-cache-info` but
`curl https://jungle.bsc.es/cachenix-cache-info` worked.

m/hut/nginx.nix

@@ -38,7 +38,7 @@ in
           proxy_redirect http:// $scheme://;
         }
         location /cache {
-          rewrite ^/cache(.*) /$1 break;
+          rewrite ^/cache/(.*) /$1 break;
           proxy_pass http://127.0.0.1:5000;
           proxy_redirect http:// $scheme://;
         }

m/module/hut-substituter.nix

@@ -0,0 +1,9 @@
+{ config, ... }:
+  # Don't add hut as a cache to itself
+  assert config.networking.hostName  != "hut";
+{
+  nix.settings = {
+    substituters = [ "https://jungle.bsc.es/cache" ];
+    trusted-public-keys = [ "jungle.bsc.es:pEc7MlAT0HEwLQYPtpkPLwRsGf80ZI26aj29zMw/HH0=" ];
+  };
+}

m/owl1/configuration.nix

@@ -8,6 +8,7 @@
     ../module/slurm-client.nix
     ../module/slurm-firewall.nix
     ../module/debuginfod.nix
+    ../module/hut-substituter.nix
   ];
 
   # Select the this using the ID to avoid mismatches

m/owl2/configuration.nix

@@ -8,6 +8,7 @@
     ../module/slurm-client.nix
     ../module/slurm-firewall.nix
     ../module/debuginfod.nix
+    ../module/hut-substituter.nix
   ];
 
   # Select the this using the ID to avoid mismatches

m/raccoon/configuration.nix

@@ -3,6 +3,7 @@
 {
   imports = [
     ../common/base.nix
+    ../module/hut-substituter.nix
   ];
 
   # Don't install Grub on the disk yet