Canonicalize binary caches with ‘/’ when one is missing

This checks if there is a trusted substituter with a slash, so
trusting https://cache.nixos.org also implies https://cache.nixos.org/
is trusted.

src/libstore/daemon.cc

@@ -215,6 +215,8 @@ struct ClientSettings
                 for (auto & s : ss)
                     if (trusted.count(s))
                         subs.push_back(s);
+                    else if (!hasSuffix(s, "/") && trusted.count(s + "/"))
+                        subs.push_back(s + "/");
                     else
                         warn("ignoring untrusted substituter '%s'", s);
                 res = subs;