[REQ] Add built-in arg for the selected build target distro

[cpuguy83]

What kind of request is this?

Improvement of existing experience

What is your request or suggestion?

e.g. When you have a build target like azlinux3/rpm the "distro" here is azlinux3.
Being able to access this in the build steps can help make some builds more versatile.
Especially if you have windows/non-linux builds in one spec.

Are you willing to submit PRs to contribute to this feature request?

• Yes, I am willing to implement it.