Add support for Ubuntu 20.04 (Focal Fossa) release

Is your feature request related to a problem? Please describe.

az cli can’t be installed on Ubuntu 20.04 using apt after referring to official method, as right after apt update user will be greeted with following error:

E: The repository 'https://packages.microsoft.com/repos/azure-cli focal Release' does not have a Release file.

As I originally pointed out in https://github.com/Azure/azure-cli/issues/9585#issuecomment-608319542, only way to install it is to resort to a hack which is to add an older Ubuntu release to /etc/apt/sources.list.d/azure-cli.list file. i.e:

deb [arch=amd64] https://packages.microsoft.com/repos/azure-cli/ disco main

Which will then lead to other problems as discovered in: https://github.com/Azure/azure-cli/issues/13030#issuecomment-615547689

Describe the solution you’d like

As someone who used to create various Debian packages and handle custom repositories. Since last Ubuntu release libffi6 is availability for is 19.04, I would simply repackage this package for 20.04 and upload it as such to https://packages.microsoft.com/repos/azure-cli repo. After adding support for focal release for the repository, you’ve solved the problem of inability to have az cli installed and all of its features fully working on >= Ubuntu 20.04.

Describe alternatives you’ve considered

I would encourage you as a company to give az cli snap more attention, as one such packaging format could’ve solve all the problem mentioned here. I had a similar problem with my Python app which I addressed by creating a snap.

Additional context

None, please inform me if you have any additional questions and/or comments.

[fengzhou-msft]

Focal package released for azure-cli 2.6.0. Please try again.

[jetzerb]

My current workaround is a pair of hacks to install the latest pre-packaged version on focal / 20.04:

# 1. Override the installer script to hardcode `eoan` package source:
curl -sL https://aka.ms/InstallAzureCLIDeb | sed 's/${CLI_REPO}/eoan/g;' | bash;

# 2. Add a symlink to make it appear that libffi 6 is installed:
cd /lib/x86_64-linux-gnu;
ln -s libffi.so.7.1.0 libffi.so.6;