ETA precision is unneeded and complicated.

[shepmaster]

Describe the problem you are trying to solve

This is what you get to stare at when updating Rust on Windows:

info: installing component 'rust-docs'
  2.8 MiB /  10.2 MiB ( 27 %)  19.2 KiB/s ETA: 6.610965304904514 min 36.65791829427087 s

Describe the solution you'd like

15 decimals of precision gets us down to femtoseconds, an utterly unneeded specificity.

Additionally, reporting the precision on the minutes and the seconds is redundant.

By nature, this calculation is an estimate and doesn't need this amount of precision. Round up to a comparatively large timeslice (5, 10 seconds) and omit the decimals on minutes:

info: installing component 'rust-docs'
  2.8 MiB /  10.2 MiB ( 27 %)  19.2 KiB/s ETA: 6m 40s

This hopefully have the nice side effect of preventing it from bouncing around a lot as well.

[nrc]

Dup of #1696, closed by #1699