Had to also bump ubuntu version 18.04->20.04,
as was getting errors running the self-compiled pyinstaller otherwise (weird...):
```
from .utils.git import get_repo_revision
ModuleNotFoundError: No module named 'PyInstaller.utils'
```
(similar to https://github.com/pyinstaller/pyinstaller/issues/4403 )
-----
Note re appimage: when trying to compile python 3.8.x on ubuntu 16.04, I am getting:
./Modules/posixmodule.c: In function ‘os_copy_file_range_impl’:
./Modules/posixmodule.c:10351:15: error: implicit declaration of function ‘copy_file_range’ [-Werror=implicit-function-declaration]
ret = copy_file_range(src, p_offset_src, dst, p_offset_dst, count, flags);
This is because ubuntu 16.04 has too old glibc.
