qt: Set DISPLAY env var when not present

Fixes web browser opening (Help > Open Mods Page, Help > Open Quickstart 
Guide)

src/yuzu/main.cpp

@@ -59,6 +59,7 @@ static FileSys::VirtualFile VfsDirectoryCreateFileWrapper(const FileSys::Virtual
 #include <QPushButton>
 #include <QShortcut>
 #include <QStatusBar>
+#include <QString>
 #include <QSysInfo>
 #include <QUrl>
 #include <QtConcurrent/QtConcurrent>
@@ -2999,6 +3000,14 @@ int main(int argc, char* argv[]) {
     chdir(bin_path.c_str());
 #endif
 
+#ifdef __linux__
+    // Set the DISPLAY variable in order to open web browsers
+    // TODO (lat9nq): Find a better solution for AppImages to start external applications
+    if (QString::fromLocal8Bit(qgetenv("DISPLAY")).isEmpty()) {
+        qputenv("DISPLAY", ":0");
+    }
+#endif
+
     // Enables the core to make the qt created contexts current on std::threads
     QCoreApplication::setAttribute(Qt::AA_DontCheckOpenGLContextThreadAffinity);
     QApplication app(argc, argv);