Can't run extension manager twice with 1.2-dev appimage
#### Steps to reproduce:
<!-- Describe what you did (step-by-step) so we can reproduce: -->
- download latest 1.2-dev appimage from https://gitlab.com/inkscape/inkscape/
- run inkscape
- Open extension manager (first time should install it)
- close and reopen the extension manager
#### What happened?
- Second attempt gives an error: `/tmp/.mount_InkscaCkqb3U/usr/bin/python3.8: can't find '__main__' module in '.'`
#### What should have happened?
- no error
#### Version info
Inkscape 1.2-dev (f2d16dd, 2021-12-14)
GLib version: 2.64.6
GTK version: 3.24.20
glibmm version: 2.64.2
gtkmm version: 3.24.2
libxml2 version: 2.9.10
libxslt version: 1.1.34
Cairo version: 1.16.0
Pango version: 1.44.7
HarfBuzz version: 2.6.4
OS version: Linux Mint 20
- Doesn't happen in 1.1.x appimage (Inkscape 1.1.1 (f36105d, 2021-12-13))
- I think the first working build where the extension manager was removed from the menu was 7d81ca390369f0f7ebb67fe741c315853e133098. It was added back in inkscape@9402e808ecb26d872ab5f0683779dc5b09a3afe8, at which point it was broken in appimage.
issue