fix Nix build and add updater

- move unnecessary toplevel files to nix/
- added patch that ignores the submodule (revert)
- add update script run by a workflow

nix/update-inputs.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env -S nix shell nixpkgs#gawk nixpkgs#git nixpkgs#moreutils -c bash
+
+# get wlroots revision from submodule
+rev=$(git submodule status | awk '{ print substr($1,2)}')
+
+# update nixpkgs to latest version
+nix flake lock --update-input nixpkgs
+
+# update wlroots to submodule revision
+nix flake lock --override-input wlroots "gitlab:wlroots/wlroots/$rev?host=gitlab.freedesktop.org"
+
+# remove "dirty" mark from lockfile
+jq < flake.lock 'del(.nodes.wlroots.original.rev)' | sponge flake.lock