Can't run extension manager twice with 1.2-dev appimage

Steps to 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@9402e808, at which point it was broken in appimage.