qml: Disable disk cache entirely

This is a compromised change. Essentially, I'm working around a
poorly-understood failure mode in Qt5, where the disk cache can
occasionally fail to invalidate, resulting in stale compiled units being
used.

This will increase application start times in favour of correctness.

src/main.cpp

@@ -140,6 +140,16 @@ int main(int argc, char *argv[])
     }
 
     QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+
+    /** QtQuick on QT5 exhibits spurious disk cache failures that cannot be
+     * resolved by a user in a trivial manner (they have to delete the cache manually).
+     * 
+     * This flag can potentially noticeably increase the start time of the application, however
+     * between this and a hard-to-detect spurious failure affecting Linux, macOS and Windows,
+     * this trade is the one most likely to result in a good experience for the widest group
+     * of users.
+     */
+    qputenv("QML_DISABLE_DISK_CACHE", "true");
 #ifdef Q_OS_WIN
     // prefer ANGLE (DirectX) over desktop OpenGL
     QCoreApplication::setAttribute(Qt::AA_UseOpenGLES);