Adds a new internal builder that makes it possible to override the
`emacsPackages` passed to our Emacs packages, which in turn makes it
possible to inject them into the emacsPackages fixpoint and use them
with features like Emacs native compilation.
Change-Id: I80dad57115c83cf5693ae6ba4e4cf3105d103d5e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3003
Tested-by: BuildkiteCI
Reviewed-by: adisbladis <adisbladis@gmail.com>
Reviewed-by: grfn <grfn@gws.fyi>
