AppImage build results in broken binary on Ubuntu 20.04

[daniel-123]

Observed symptoms:

• Running the AppImage results in segmentation fault
• Size of the broken AppImage is smaller by few MB
• Broken 20.04 AppImage contains usr/lib/libctf-nobfd.so.0 which isn't present in 18.04

[daniel-123]

While doing research on #454 I discovered that environment being used to build AppImage effectively sets the minimum versions of low level libraries. Thus AppImage should always be built on oldest environment we want to support.