Disallow running CLI with root privileges (#32583)

In #31331, I made a change that prevents Zed from running with root
privileges, but I forgot about the CLI.
So if you run the CLI without the `--foreground` flag, it just freezes
without any messages. This PR fixes that.

Release Notes:

- N/A

crates/zed/Cargo.toml

@@ -88,7 +88,7 @@ markdown_preview.workspace = true
 menu.workspace = true
 migrator.workspace = true
 mimalloc = { version = "0.1", optional = true }
-nix = { workspace = true, features = ["pthread", "signal", "user"] }
+nix = { workspace = true, features = ["pthread", "signal"] }
 node_runtime.workspace = true
 notifications.workspace = true
 outline.workspace = true