feat(emacs): Add 'withLocalEmacs' override function

This function makes it possible to build an Emacs instance that,
instead of launching an Emacs built by Nix, configures an Emacs
already present on the system to use the packages built by Nix.

This **requires** that the versions of the two Emacsen (i.e. the one
used by Nix to build and the one used to run the packages) are kept in
sync, otherwise byte-code incompatibilities may lead to undefined
behaviour.

tools/emacs/default.nix

@@ -101,4 +101,16 @@ in lib.fix(self: l: f: third_party.writeShellScriptBin "tazjins-emacs" ''
     # Call withLocalConfig with the path to a *folder* containing a
     # `local.el` which provides local system configuration.
     withLocalConfig = confDir: self confDir f;
+
+    # Build a derivation that uses the specified local Emacs (i.e.
+    # built outside of Nix) instead
+    withLocalEmacs = emacsBin: third_party.writeShellScriptBin "tazjins-emacs" ''
+      exec ${emacsBin} \
+        --debug-init \
+        --no-site-file \
+        --no-init-file \
+        --directory ${(tazjinsEmacs f).deps}/share/emacs/site-lisp \
+        --directory ${./config} ${if l != null then "--directory ${l}" else ""} \
+        --eval "(require 'init)" $@
+    '';
   }) null identity