flake.nix: use `--watch=always` for tailwind watcher

This forces the watcher to stay alive when running non-interactively
(e.g. when run from within Tilt).

Signed-off-by: Isaac Corbrey <[e-mail]>

flake.nix

···
       tailwind-watcher =
         pkgs.writeShellScriptBin "run"
         ''
-          ${pkgs.tailwindcss}/bin/tailwindcss -w -i input.css -o ./appview/pages/static/tw.css
+          ${pkgs.tailwindcss}/bin/tailwindcss --watch=always -i input.css -o ./appview/pages/static/tw.css
         '';
     in {
       fmt = {