Disable system proxy support on Linux

Reading system proxy on Linux reads to massive crashes in system libraries.

Since those crashes are unreproducible in development environment, it's not possible to find the root cause.

Workarounds telegramdesktop#27482.

Telegram/SourceFiles/core/sandbox.cpp

@@ -33,6 +33,7 @@ For license and copyright information please follow this link:
 #include <QtGui/QSessionManager>
 #include <QtGui/QScreen>
 #include <QtGui/qpa/qplatformscreen.h>
+#include <ksandbox.h>
 
 namespace Core {
 namespace {
@@ -517,8 +518,10 @@ void Sandbox::refreshGlobalProxy() {
 		|| proxy.type == MTP::ProxyData::Type::Http) {
 		QNetworkProxy::setApplicationProxy(
 			MTP::ToNetworkProxy(MTP::ToDirectIpProxy(proxy)));
-	} else if (!Core::IsAppLaunched()
-		|| Core::App().settings().proxy().isSystem()) {
+	} else if ((!Core::IsAppLaunched()
+		|| Core::App().settings().proxy().isSystem())
+		// this works stable only in sandboxed environment where it works through portal
+		&& (!Platform::IsLinux() || KSandbox::isInside() || cDebugMode())) {
 		QNetworkProxyFactory::setUseSystemConfiguration(true);
 	} else {
 		QNetworkProxy::setApplicationProxy(QNetworkProxy::NoProxy);