CI Makefile should be considered an end user interface

To build a neat bindist, the only simple way is to use the top-level Makefile, but that is a little awkwardly intertwined with assumptions about CI (e.g. removal of GHC versions to work around space constraints).

The build system to build binaries should be considered an end user interface to allow:

* distributors to easily build their own bindists
* end users building a bindist for their own purposes or to share for debugging purposes
* GHCup to be able to build proper HLS bindists instead of ad-hoc binaries

Start with the top-level Makefile and inspect how its bindist targets depend on CI assumptions, including removal of GHC versions for space constraints. Define a build-system interface that distributors, end users, and GHCup can use to build proper HLS bindists, then verify that the resulting process supports those stated use cases.

Written by the indexing model from the issue text.