Workaround broken devenv up in devshell

As per https://github.com/cachix/devenv/issues/756, devenv up is broken
in the standard devshell because it can't find the devenv-up package. By
setting this line to ....procfileScript we provide devenv with the
information it needs to reactivate devenv up

Change-Id: Ie35b461c0aca899fcf366afa204273c51e3a560b
Reviewed-on: https://git.clicks.codes/c/Caramels/Lotting/+/344
Tested-by: Samuel Shuert <coded@clicks.codes>
Reviewed-by: Samuel Shuert <coded@clicks.codes>
diff --git a/flake.nix b/flake.nix
index e7f363c..7bf9252 100644
--- a/flake.nix
+++ b/flake.nix
@@ -14,6 +14,7 @@
       pkgs = nixpkgs.legacyPackages."x86_64-linux";
     in
     {
+      packages.x86_64-linux.devenv-up = self.devShell.x86_64-linux.config.procfileScript;
       devShell.x86_64-linux = devenv.lib.mkShell {
         inherit inputs pkgs;
         modules = [