flake.nix: make the build process more robust

flake.nix

@@ -18,10 +18,12 @@
         pname = "vkane.cz";
         version = "";
         src = self;
+        # the theme is included as a submodule, and with nix it is an alchemy whether it is somehow included or not (in which case the empty parent directory might be missing)
         preConfigure = ''
           rm --dir --force themes/hermit
+          mkdir -p themes
           ln --symbolic '${hermit}' themes/hermit
-        ''; # TODO
+        '';
         buildPhase = ''
           '${pkgs.hugo}'/bin/hugo --buildExpired --buildFuture --destination "$out" --minify --printPathWarnings --printI18nWarnings
         '';