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.

default.nix

@@ -340,6 +340,8 @@ let
   # Use runCommand, not runCommandLocal, because makeBinaryWrapper pulls in a compiler.
   doomEmacs = runCommand "doom-emacs" {
     nativeBuildInputs = [ makeBinaryWrapper ];
+    # Expose this so we can push it to cachix.
+    inherit emacsWithPackages;
   } ''
   if [[ -z "${profileName}" ]]; then
     profileArgs=()