flake: Keep `crates/**/*.txt` files for build — heartwood

Fix builds broken due to include_str! failing to read a text file.

         ]
         ||
         # Default filter from crane (allow .rs files)
         (rustup.craneLib.filterCargoSources path type);
         (rustup.craneLib.filterCargoSources path type)
         ||
         # *.txt files might be included with `include_str!`
         (builtins.isList (builtins.match ''/?crates/(.*/.*\.txt)'' path));
       src = lib.cleanSourceWith {
         src = ./.;