bg: Fix reuse opaque struct across bridges issue

Author: clemenswasser

fish-rust/src/builtins/bg.rs

@@ -44,7 +44,8 @@ fn send_to_bg(
         job.command().from_ffi()
     ));
 
-    job.ffi_group().set_is_foreground(false);
+    unsafe { std::mem::transmute::<&ffi::job_group_t, &crate::job_group::JobGroup>(job.ffi_group()) }
+        .set_is_foreground(false);
 
     if !job.ffi_resume() {
         return STATUS_CMD_ERROR;