Fix a BorrowMut error when stdout is closed

There was one location in the job queue where the shell is borrowed
twice by accident, so instead hold the same borrow over both locations.

Closes #9220

Author: alexcrichton

src/cargo/core/compiler/job_queue.rs

@@ -748,13 +748,14 @@ impl<'cfg> DrainState<'cfg> {
             }
         }
         if cx.bcx.build_config.emit_json() {
+            let mut shell = cx.bcx.config.shell();
             let msg = machine_message::BuildFinished {
                 success: error.is_none(),
             }
             .to_json_string();
-            if let Err(e) = writeln!(cx.bcx.config.shell().out(), "{}", msg) {
+            if let Err(e) = writeln!(shell.out(), "{}", msg) {
                 if error.is_some() {
-                    crate::display_error(&e.into(), &mut cx.bcx.config.shell());
+                    crate::display_error(&e.into(), &mut shell);
                 } else {
                     return Some(e.into());
                 }