Manual: Update chapter on remote builds

Alos add a command "nix ping-store" to make it easier to see if Nix
can connect to a remote builder (e.g. 'nix ping-store --store
ssh://mac').

src/nix/ping-store.cc

@@ -0,0 +1,35 @@
+#include "command.hh"
+#include "shared.hh"
+#include "store-api.hh"
+
+using namespace nix;
+
+struct CmdPingStore : StoreCommand
+{
+    std::string name() override
+    {
+        return "ping-store";
+    }
+
+    std::string description() override
+    {
+        return "test whether a store can be opened";
+    }
+
+    Examples examples() override
+    {
+        return {
+            Example{
+                "To test whether connecting to a remote Nix store via SSH works:",
+                "nix ping-store --store ssh://mac1"
+            },
+        };
+    }
+
+    void run(ref<Store> store) override
+    {
+        store->connect();
+    }
+};
+
+static RegisterCommand r1(make_ref<CmdPingStore>());