Environment variable loading is broken in sandboxed mode when Nushell is the default shell

### Log snippet

```
2025-11-15T09:13:00-06:00 ERROR [crates/fs/src/fs_watcher.rs:216] Error { kind: Io(Os { code: 22, kind: InvalidInput, message: "Invalid argument" }), paths: ["/home/harper/cports/contrib/contrib-placeholder-dbg"] }
2025-11-15T09:13:00-06:00 ERROR [crates/project/src/environment.rs:227] capturing shell environment with "/app/bin/host-spawn"

Caused by:
0: Failed to deserialize environment variables from json: {env_output}
1: EOF while parsing a value at line 1 column 0
```

### Analysis

I believe the problem is that (as that snippet reveals) zed thinks the shell is `host-spawn`, not my actual shell (`nushell`). As of https://github.com/zed-industries/zed/pull/35002, zed special-cases how it handles nushell, and it's not able to do so because it's confused about what the actual shell is.