Progress bar hopelessly wrong for Rhythm Track and other Nyquist generators

[SteveDaulton]

Describe the bug
Audacity 3.0.3 and later, the progress bar is hopelessly wrong for Rhythm Track and other Nyquist generate effects.
Most obvious with Rhythm Track as it is the only shipped Nyquist generate type plug-in to produce a long output, but affects all Nyquist generate type plug-ins that take longer than a few seconds to complete.

To Reproduce
Steps to reproduce the behavior:

1. Generate a long (> 1 hour) Rhythm Track
2. Observe the progress bar, and note that "Remaining Time" continuously increases until the effect completes.

Expected behavior
An approximate indication of the time remaining

Screenshots
This shows the progress bar as Rhythm Track completes a 10 hour track in 38 seconds

Additional information (please complete the following information):

• OS: Xubuntu 20.04 and Windows 10 (macOS not tested)
• Version: Audacity 3.0.3 and later

Additional context
Because Audacity cannot know how much audio a Nyquist generator will produce, a plug-in may set the LEN variable to give Audacity a hint as to how much audio will be produced. This in turn would allow the progress bar to operate in a reasonably sane way. Unfortunately this broke between Audacity 3.0.2 and 3.0.3.

Example:

;type generate
(setf LEN (* 3600 *sound-srate*))
(osc 72 3600)

In Audacity 3.0.2 and earlier, the above code in the Nyquist prompt will display a sane progress bar as it generates a 1 hour long sine tone.
In Audacity 3.0.3 and later the progress bar displays a continuously increasing "Remaining Time" while progress bar is stuck at zero.

See also: #1855