When I designed `mkShell`, I didn't have a good idea of what the output
should look like and so decided to make the build fail. In practice,
this causes quite a bit of confusion and complications because now the
shell cannot be part of a normal package set without failing the CI as
well.

This commit changes that build phase to record all the build inputs in a
file. That way it becomes possible to build it, makes sure that all the
build inputs get built as well, and also can be used as a GC root.
(by applying the same trick as NixOS#95536).

The documentation has also been improved to better describe what mkShell
does and how to use it.

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

Co-authored-by: Robert Hensing <roberth@users.noreply.github.com>

Co-authored-by: Cole Helbling <cole.e.helbling@outlook.com>