Create a .env file with all the environment variables

Overview

We currently use _common.gitlab-ci.yml as our single source of truth for all the software version we install and the environments variables we need for our build tools, we also have some platform-specific ones such as windows.

It's awesome that we have everything in 1 place where we can update software, but the problem here is that these are defined in CI, and not in the user's environment. So if a developer needs to build the helper image locally or some they have to find that variable from the .gitlab-ci.yml file, which is less then ideal. Sometimes we also end up adding defaults values to variables which can get outdated. It's also very tedious for things like building the Windows helper image where you have to define a lot of variables and copy paste them.

Proposal

Instead of having .gitlab-ci.yml as the single source of truth for these environment variables, we need to see if should have a .env file where both CI and our makefiles should be able to source them and pick up the default values. Like this both the CI and our development have 1 SSOT and doesn't require us to have default values nor copy pasting when we want to build something.

/cc @pedropombeiro @akohlbecker since you both brought up this pain in the past