Fix nginx /cache regex

`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://;
         }