Nixpkgs PR with jpm/janet fix

I found even with my PR, jpm isn't working because it doesn't have permissions to nix store. Struggled for few hours, checked how jpm internally use all those options, checked how other package managers like luarocks handle this issue... And end up leaving it. I can't fix this because I can't assure that $HOME will be available when jpm is installed. Instead, I found --tree flag and JANET_TREE environment variable which works similar to luarocks's --tree flag. This allows me to make it working with jpm build --tree=$HOME/.jpm.

Janet prompt?

With all those jpm fix, I made small script to pretty-print jj-vcs information in prompt. But... it's slow as a turtle.

I think I can fix this by implementing asynchronous prompt refresh, but that's bit tedious in zsh and I'm not planning switching to fish yet.