Set export_linear_aec_output on multichannel AEC config

When linear AEC output export is enabled via the `--linear_aec_output` flag,
`export_linear_aec_output` was previously only enabled on the mono
`EchoCanceller3Config`, leaving the builder's multichannel config unset.
This caused config compatibility validation in `ConfigSelector` to fail.
This CL ensures `export_linear_aec_output` is also enabled on
`echo_canceller_multichannel_config` when present.

Bug: webrtc:42230301
Change-Id: I7e4f42089e2862b56e7ca070e94c96dd786d8b78
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/504741
Reviewed-by: Sam Zackrisson <saza@webrtc.org>
Commit-Queue: Per Åhgren <peah@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#48661}