Fix bench build: use PIDs instead of job specs for wait

wait %1 and %2 require job control (set -m), which is disabled in
non-interactive shells. nick-fields/retry runs commands in a
non-interactive shell, causing 'no such job' (exit 127) even when
both builds succeed. Use $! PIDs instead.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

.github/workflows/bench.yml

@@ -108,8 +108,10 @@ jobs:
           timeout_minutes: 480
           command: |
             (cd pr     && ${{ matrix.build_script }}) &
+            pid1=$!
             (cd master && ${{ matrix.build_script }}) &
-            wait %1 && wait %2
+            pid2=$!
+            wait $pid1 && wait $pid2
           on_retry_command: |
             (cd pr     && ./mfc.sh clean) &
             (cd master && ./mfc.sh clean) &