修复系统代理 JVM 参数格式 (#5265)

Co-authored-by: xirren <xirren@outlook.com>

HMCLCore/src/main/java/org/jackhuang/hmcl/launch/DefaultLauncher.java

@@ -172,7 +172,7 @@ public class DefaultLauncher extends Launcher {
 
             Proxy.Type proxyType = options.getProxyType();
             if (proxyType == null) {
-                res.addDefault("-Djava.net.useSystemProxies", "true");
+                res.addDefault("-Djava.net.useSystemProxies=", "true");
             } else {
                 String proxyHost = options.getProxyHost();
                 int proxyPort = options.getProxyPort();