Improve Decision image generation

[ahal]

I'd like to solidify the Decision image generation, making it more robust and reproducible. Namely:

1. Change the base from Ubuntu 20.04 to Debian 12.
2. Adopt package tasks (as seen in Gecko), so we can explicitly specify versions of certain tools (like Mercurial).
3. Provide a base layer below the Decision image so we don't update the world everytime it is re-generated.

I think we'll likely end up with something similar to what Gecko has, where there is:

1. A debian-raw image that contains some bare-bones setup to add apt-sources from Taskcluster
2. A debian-packages image which is only used by the packages tasks.
3. packages tasks to pin specific versions of various tools needed (possibly just Mercurial to start)
4. A debian-base image that depends on debian-raw + the packages task, and sets up packages + other debian packages needed.
5. A decision image that sits on top of debian-base and only layers on Taskgraph itself.

I consider this work to be a pre-requisite to #46 as it will allow us to update only Taskgraph in the image, leaving everything else identical. This should make Decision images much more robust and reproducible. The debian-base image can also be re-used for other tasks or projects.