[docs] fix RTD builds

[StrikerRUS]

Collecting package metadata: ...working... done
Solving environment: ...working... failed


PackagesNotFoundError: The following packages are not available from current channels:

  - r-base==4.0.3=hc603457_2

Current channels:

  - https://conda.anaconda.org/conda-forge/linux-64
  - https://conda.anaconda.org/conda-forge/noarch
  - https://repo.anaconda.com/pkgs/main/linux-64
  - https://repo.anaconda.com/pkgs/main/noarch
  - https://repo.anaconda.com/pkgs/free/linux-64
  - https://repo.anaconda.com/pkgs/free/noarch
  - https://repo.anaconda.com/pkgs/r/linux-64
  - https://repo.anaconda.com/pkgs/r/noarch

To search for alternate channels that may provide the conda package you're
looking for, navigate to

    https://anaconda.org

and use the search bar at the top of the page.


Could not find conda environment: r_env
You can list all discoverable environments with `conda info --envs`.

[StrikerRUS]

[jameslamb]

oh wow does this mean that a build of r-base was taken down??

[StrikerRUS]

@jameslamb

oh wow does this mean that a build of r-base was taken down??

Who knows?.. I guess they found that build critically corrupted and replaced it with a new one instead of simply upload new version.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.