elpaBuild: Delay src evaluation so it can be overridden

This change allows ELPA packages to have their src attribute updated
by overrideAttrs.  Without this change the installPhase references the
original src attribute and overriding is not possible.

pkgs/build-support/emacs/elpa.nix

@@ -28,7 +28,7 @@ import ./generic.nix { inherit lib stdenv emacs texinfo writeText; } ({
 
     emacs --batch -Q -l ${./elpa2nix.el} \
         -f elpa2nix-install-package \
-        "${src}" "$out/share/emacs/site-lisp/elpa"
+        "$src" "$out/share/emacs/site-lisp/elpa"
 
     runHook postInstall
   '';