Disable qml diskcache by default

Since this causes crashes on some qml module updates.

Fixes #1383
Fixes #1132

man/nheko.1.adoc

@@ -272,6 +272,12 @@ Hide text and set an optional spoiler warning. Note that Nheko does not display
 the spoiler warning currently. HTML output:
 `<span data-mx-spoiler="spoiler">text</span>`
 
+== ENVIRONMENT
+
+*NHEKO_ALLOW_QML_DISK_CACHE*::
+Nheko by default disables the qml disk cache to prevent crashes. This allows
+you to reenable it at your own risk.
+
 == FILES
 
 *Configuration file*::

src/main.cpp

@@ -157,6 +157,12 @@ main(int argc, char *argv[])
     QCoreApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
     QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
 
+    // Disable the qml disk cache by default to prevent crashes on updates. See
+    // https://github.com/Nheko-Reborn/nheko/issues/1383
+    if (qgetenv("NHEKO_ALLOW_QML_DISK_CACHE").size() == 0) {
+        qputenv("QML_DISABLE_DISK_CACHE", "1");
+    }
+
     // this needs to be after setting the application name. Or how would we find our settings
     // file then?
 #if !defined(Q_OS_MACOS)