Use either authlib-injector or system properties, not both

Resolves #154

In 1.20+, authlib-injector wants to intercept
https://api.minecraftservices.com/publickeys [0], but if we override the
services server using -Dminecraft.api.services.host, the request does
not get intercepted.

That's not an issue for API servers that implement the /publickeys
route, but Ely.by does not. A decent solution is to not set these
-Dminecraft.api.*.host system properties when authlib-injector is
available, since authlib-injector expects authlib to make requests to
Mojang's API servers.

[0] yushijinhun/authlib-injector@18a0ce2

launcher/minecraft/MinecraftInstance.cpp

@@ -544,11 +544,7 @@ QStringList MinecraftInstance::processAuthArgs(AuthSessionPtr session) const
 {
     QStringList args;
     if (session->uses_custom_api_servers) {
-        args << "-Dminecraft.api.env=custom";
-        args << "-Dminecraft.api.auth.host=" + session->auth_server_url;
-        args << "-Dminecraft.api.account.host=" + session->account_server_url;
-        args << "-Dminecraft.api.session.host=" + session->session_server_url;
-        args << "-Dminecraft.api.services.host=" + session->services_server_url;
+        bool using_authlib_injector = false;
         auto agents = m_components->getProfile()->getAgents();
         for (auto agent : agents) {
             if (agent->library()->artifactPrefix() == "moe.yushi:authlibinjector") {
@@ -562,9 +558,18 @@ QStringList MinecraftInstance::processAuthArgs(AuthSessionPtr session) const
                 if (session->authlib_injector_metadata != "") {
                     args << "-Dauthlibinjector.yggdrasil.prefetched=" + session->authlib_injector_metadata;
                 }
+                using_authlib_injector = true;
                 break;
             }
         }
+        if (!using_authlib_injector) {
+            qDebug() << "authlib-injector not found, setting -Dminecraft.api.*.host system properties.";
+            args << "-Dminecraft.api.env=custom";
+            args << "-Dminecraft.api.auth.host=" + session->auth_server_url;
+            args << "-Dminecraft.api.account.host=" + session->account_server_url;
+            args << "-Dminecraft.api.session.host=" + session->session_server_url;
+            args << "-Dminecraft.api.services.host=" + session->services_server_url;
+        }
     }
     return args;
 }