style: apply nightly rustfmt import grouping across the workspace

rustfmt.toml sets group_imports, imports_granularity and imports_layout,
which are nightly-only; agents formatted with stable rustfmt, which
ignored them. This is a formatting-only pass with `cargo +nightly fmt`.

Test Plan:
- cargo clippy --workspace --all-targets -- -D warnings
- cargo test --workspace

crates/ghostty-os/src/env.rs

@@ -1,5 +1,7 @@
-use std::collections::HashMap;
-use std::ffi::{OsStr, OsString};
+use std::{
+    collections::HashMap,
+    ffi::{OsStr, OsString},
+};
 
 use crate::flatpak::is_flatpak;
 
@@ -214,9 +216,10 @@ pub fn getenv_not_empty(key: &str) -> Option<String> {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
     use std::os::unix::ffi::OsStringExt;
 
+    use super::*;
+
     #[test]
     fn env_map_preserves_non_utf8() {
         let mut env = EnvMap::new();