fix(build): prefix load paths instead of set

Allows callers to add their own load paths. See example config for how one can
now prefix `EMACSNATIVELOADPATH` for a byte-compiled early-init.el/init.el:

https://github.com/jordanisaacs/emacs-config/blob/c3ba675888a8334f403e4304828af221fa0d5e55/flake.nix#L100

For some reason emacs expects ":" in a single value `EMACSLOADPATH`.  However,
`--prefix` escapes ":" values. To get around this, use `--set-default
EMACSLOADPATH ":"` which results in the following when no caller above adds
their own load paths: `EMACSLOADPATH=$siteLisp::` which emacs accepts.

pkgs/emacs/wrapper.nix

@@ -182,9 +182,10 @@ in
         ${lib.optionalString (length executablePackages > 0) "--prefix PATH : ${lib.escapeShellArg (lib.makeBinPath executablePackages)}"} \
         --prefix INFOPATH : ${emacs}/share/info:$out/share/info:${infoPath} \
         ${
-      lib.optionalString nativeComp "--set EMACSNATIVELOADPATH $nativeLisp:$nativeLoadPath"
+      lib.optionalString nativeComp "--prefix EMACSNATIVELOADPATH : $nativeLisp:$nativeLoadPath"
     } \
-        --set EMACSLOADPATH "$siteLisp:"
+        --set-default EMACSLOADPATH ":" \
+        --prefix EMACSLOADPATH : "$siteLisp"
       fi
     done
   ''