fix "Package lib32gcc1 is not available"

[RoyCLR]

Problem

Running docker image build today (Sep2022):

docker build ursim/e-series -t myursim_eseries --build-arg VERSION=5.12.4.1101661 --build-arg URSIM="https://s3-eu-west-1.amazonaws.com/ur-support-site/174207/URSim_Linux-5.12.4.1101661.tar.gz

fails with:

Package lib32gcc1 is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
However the following packages replace it:
  lib32gcc-s1

Possible cause

Quoting GameServerManagers/LinuxGSM#3093

Ubuntu has renamed lib32gcc1 to lib32gcc-s1

but the URSim install script seems to check for the exact version to install its dependencies

Proposed fix in this PR

change base image from ubuntu:latest to ubuntu:20.04

[ArdooTala]

Worked for me. Thank you.