Run ReplayGain analysis after conversion

```
replaygain: analyzing Adele - 21
replaygain: ReplayGain error: ReplayGain backend failed for some tracks in album Adele - 21
```

running 1.3.16 in a docker container.

I convert FLAC to MP3 and replaygain executes before the conversion, so I am not sure if the issue is lack of support for FLAC or something else. I would like to think that replaygain should execute against the MP3 after conversion, but that is just a thought.

I am able to run replay gain after the import completes and that works fine, so the issue is only with flac on the initial import.

Reproduce the initial import in a Docker container using beets 1.3.16 with FLAC input converted to MP3, comparing it with ReplayGain run after import. Trace the import conversion and ReplayGain ordering, then confirm that analysis succeeds against the converted MP3 without breaking the working post-import behavior.

Written by the indexing model from the issue text.