Run conda clean after conda update if cleanup = true

[maxkapur]

What does this PR do

Conda provides a conda clean subcommand, sort of like apt autoremove, that automatically removes used unused packages, tarballs, and so on:

https://docs.conda.io/projects/conda/en/latest/commands/clean.html

This PR updates the Conda topgrade step to run conda clean --all if cleanup = true is set in the topgrade config. It adds --yes if topgrade is run with -y.

Standards checklist

• The PR title is descriptive
• I have read CONTRIBUTING.md
• Optional: I have tested the code myself
• (n/a) If this PR introduces new user-facing messages they are translated

For new steps

• Optional: Topgrade skips this step where needed
• Optional: The --dry-run option works with this step
• Optional: The --yes option works with this step if it is supported by
  the underlying command

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.

[maxkapur]

Notes:

• conda clean also provides a --dry-run flag that could reflect whether topgrade itself was run with --dry-run. I haven't implemented this because I noticed that --dry-run is also not implemented for the conda update step anyway. I might implement --dry-run in a separate PR.
• I have never contributed to this repo before and seldom use rust so please forgive any silly mistakes 😅