can not use `import <nixpkgs>` on update machine as `<nixpkgs>` does not
exist. therefore cd to the `warp-terminal` directory and use the
relative path to nixpkgs root.
https://r.ryantm.com/log/warp-terminal/2024-03-19.log
use fetchurl to calculate hash in update.sh to workaround a bug in
nix-prefetch-url.
nix-prefetch-url seems to be uncompressing and then calculating the hash
when downloading the new warp-terminal. this results in hash mismatch
errors as fetchurl calculates the hash using the compressed archive.
