fix(appimage): move VFS plugins to AppDir

fix(appimage): move VFS plugins to AppDir

This allows the AppImage build to find and load the experimental VFS
plugins, and not crash with:

    [ critical plugins /home/user/src/common/vfs.cpp:250 ]: Could not load plugin: not existent or bad metadata "nextclouddevsync_vfs_suffix"
    [ fatal default /home/user/src/gui/folderman.cpp:326 ]: Could not load plugin
    [1]    72936 IOT instruction (core dumped)  ./nextclouddev-master-x86_64.AppImage

Fixes #7135
Fixes #7194
Fixes #7465

Signed-off-by: Jyrki Gadinger <nilsding@nilsding.org>