Save off pre-built versions of git for different Ubuntu/Debian distros in GH packages

[Chuxel]

Currently there is a script to build git from source that is used in codespaces-linux. It is not being used in other pre-built images because of the time it takes to build. For Ubuntu images, we could use a PPA copy, but this does not work for Debian based images.

Instead we could create a CI job that periodically builds for git for Debian 9, 10, Ubuntu 18.04, and 20.04 (separately) and stores them in GH packages for this repo. By doing so for each distro, we can avoid the perf hit along with distro library mismatches. (Library mismatches delayed the release of a codespaces-linux image update - pre-built versions of Python for Debian 9 didn't work on Ubuntu 20.04).

We should set this up in such a way that other CLIs/utilities could be built the same way.