CI: micro-optimize cachix build

The doom profile is a non-negligible fraction of the cachix binary size,
that will only get worse (any dependency change will trigger a rebuild),
and caching them is essentially useless.

Expose emacsWithPackages so we can push just that. Skip
emacsWithPackages itself and push just the -deps derivation while we're
at it: caching either emacsWithPackages or -deps is similarly useless,
but they are a bit smaller.

When I say "non-negligible", it's about 1 MiB per "full" profile: much
larger than most individual libraries but not actually large. So this is
really a micro-optimization.

flake.nix

@@ -111,12 +111,13 @@
         }).doomEmacs;
         # TODO: cache more packages, cache for more Emacsen.
         cachix-packages = pkgs.linkFarm "unstraightened-cachix-packages" {
+          inherit doomemacs;
           full-emacs29 = (doomFromPackages pkgs {
             emacs = pkgs.emacs29;
             doomDir = ./doomdirs/minimal;
             doomLocalDir = "~/.local/share/nix-doom-unstraightened";
             full = true;
-          }).doomEmacs;
+          }).doomEmacs.emacsWithPackages.deps;
         };
       });
       overlays.default = final: prev: {