notmuch: fix notmuch-emacs-mua

Before this patch, notmuch-emacs-mua would try to call emacs in PATH,
 which would obviously fail on systems with no emacs. This fixes that,
 while still keeping the capability to override your emacs' path.

pkgs/applications/networking/mailreaders/notmuch/default.nix

@@ -42,6 +42,10 @@ stdenv.mkDerivation rec {
 
     substituteInPlace lib/Makefile.local \
       --replace '-install_name $(libdir)' "-install_name $out/lib"
+
+    substituteInPlace emacs/notmuch-emacs-mua \
+      --replace 'EMACS:-emacs' 'EMACS:-${emacs}/bin/emacs' \
+      --replace 'EMACSCLIENT:-emacsclient' 'EMACSCLIENT:-${emacs}/bin/emacsclient'
   '';
 
   configureFlags = [ "--zshcompletiondir=${placeholder "out"}/share/zsh/site-functions" ];