Wrong length of the output file

### Bug description

Muted tracks contribute to the exported audio length.

### Steps to reproduce

1. Import/generate some audio on Track 1
2. Add another Track 2 with audio longer than on Track 1
3. Mute Track 2
4. Export
5. Check the length of the exported file

### Expected behavior

It matches the audio length on Track 1.

### Actual behavior

It matches the project's total audio length, even with muted Track 2.

### Audacity Version

Audacity 4.0 beta 3

### Operating system

OS independent

Reproduce the export in Audacity 4.0 beta 3 with a muted Track 2 that is longer than Track 1, then trace the export length calculation. Confirm the fix by checking that the exported file length follows audible tracks rather than the muted track or total project length.

Written by the indexing model from the issue text.