Move uriSchemes to *StoreConfig

It is a property of the configuration of a store --- how a store URL is
parsed into a store config, not a store itself.

Progress towards #10766

src/libstore/http-binary-cache-store.hh

@@ -15,6 +15,15 @@ struct HttpBinaryCacheStoreConfig : virtual BinaryCacheStoreConfig
         return "HTTP Binary Cache Store";
     }
 
+    static std::set<std::string> uriSchemes()
+    {
+        static bool forceHttp = getEnv("_NIX_FORCE_HTTP") == "1";
+        auto ret = std::set<std::string>({"http", "https"});
+        if (forceHttp)
+            ret.insert("file");
+        return ret;
+    }
+
     std::string doc() override;
 };