Let's try adding install phase steps to phanpy

pkgs/phanpy/default.nix

@@ -25,6 +25,12 @@ buildNpmPackage rec {
   # The prepack script runs the build script, which we'd rather do in the build phase.
   npmPackFlags = ["--ignore-scripts"];
 
+  installPhase = ''
+    mkdir $out
+    npm run build
+    cp dist/index.html $out
+  '';
+
   meta = with lib; {
     description = "Minimalistic opinionated Mastodon web client.";
     homepage = "https://phanpy.social/";