nix: Allow auto-update in the devshell (#26911)

Our direnv integration was making zed refuse to auto-update when you had
the zed repo open with the devshell active. This was happening even when
you used a non-nix build of zed, which actually should be able to
auto-update.

I'm a bit unsure of why we check for the `ZED_UPDATE_EXPLANATION` env
var [both at build time _and_ at
runtime](https://github.com/zed-industries/zed/blob/2828dcb67b0427bb64dd7c350896673ff6a14782/crates/auto_update/src/auto_update.rs#L149),
but I can see an argument for why people might want that so I'll just do
the less intrusive change for now and leave the var out of the devshell.

Release Notes:

- N/A