Disallow running Zed with root privileges (#31331)

This will fix a lot of weird problems that are based on file access
issues.

As discussed in
https://github.com/zed-industries/zed/pull/31219#issuecomment-2905371710,
for now it's better to just prevent running Zed with root privileges.

Release Notes:

- Explicitly disallow running Zed with root privileges

---------

Co-authored-by: Peter Tripp <peter@zed.dev>

crates/zed/Cargo.toml

@@ -85,7 +85,7 @@ markdown_preview.workspace = true
 menu.workspace = true
 migrator.workspace = true
 mimalloc = { version = "0.1", optional = true }
-nix = { workspace = true, features = ["pthread", "signal"] }
+nix = { workspace = true, features = ["pthread", "signal", "user"] }
 node_runtime.workspace = true
 notifications.workspace = true
 outline.workspace = true