Nix promises to run us in an (empty) temporary directory. So we should
not need mktemp, we can just use deterministic names.
Our temporary directory should already not be leaking into the
output (and we enforce that for the profile), so this should just be
cleanup that makes debugging slightly easier.
We were only pulling in autoloads files for packages installed via
packages.el. This was not that noticeable because all pinned packages
without custom derivations end up installed that way, but it does affect
extra packages (including the one pointed out in #11).
Try to fix this by including autoloads files in the top-level site-lisp
directory from emacsWithPackages.
The amount of shell in non-shell files was making me uncomfortable,
and two of these previously contained awkward `''$` escapes.
Apart from forcing one more step to run locally, this is just moving
code around.
