[ci] remove conda from R CI jobs

[jameslamb]

On #3405 (comment), I see the Windows CI jobs on GitHub Actions failing with this output

==> WARNING: A newer version of conda exists. <==
  current version: 4.8.3
  latest version: 4.9.2

Please update conda by running

    $ conda update -n base -c defaults conda

#
# To activate this environment, use
#
#     $ conda activate test
#
# To deactivate an active environment, use
#
#     $ conda deactivate

I'm worried that this is another case where programs writing to stderr in GitHub Actions Windows jobs causes those jobs to fail. I don't know if this recently started failing because of software updates in GitHub Actions images (https://github.com/actions/virtual-environments/commits/main) or something else.

This PR proposes just updating conda to the latest version every time, to hopefully avoid this warning.

[jameslamb]

UPDATE I see the same failures on the Windows jobs in #3515 too:https://github.com/microsoft/LightGBM/pull/3515/checks?check_run_id=1408108173

But not on builds from last night. https://github.com/microsoft/LightGBM/actions/runs/365273180

So whatever changed, I think it was in the last 24 hours.

[jameslamb]

Ok, that did not work. https://github.com/microsoft/LightGBM/pull/3573/checks?check_run_id=1409528379

Trying to upgrade to the action to 2.0 next.

[jameslamb]

Seems like the combination of always updating conda + using v2 of this action fixes it

[jameslamb]

@StrikerRUS let's move #3405 (comment) here, since #3405 already has a lot of comments.

That's a good point! There is actually an r-lib/setup-pandoc action we could try: https://github.com/r-lib/actions/tree/master/setup-pandoc

Let me try on my fork tonight. If that works, I'll just change this PR.

[jameslamb]

Seems ok on my fork!

https://github.com/jameslamb/LightGBM/pull/38/checks?check_run_id=1409970898

I just pushed some changes that remove conda completely for R jobs

[jameslamb]

I had to move this up so it happens before conda create

[jameslamb]

@StrikerRUS I was torn between doing this != thing or just adding this before the conda steps:

if [[ "${TASK:0:9}" != "r-package" ]]; then
    exit 0
fi

Do you have a strong preference?

[StrikerRUS]

@jameslamb

or just adding this before the conda steps:

I think the second way will be more readable.

[StrikerRUS]

Never mind! I changed my mind 😄

[jameslamb]

haha ok. Easy to change if you ever want to.

[jameslamb]

I've requested a new approval, @StrikerRUS , since I totally changed this PR since the last time you reviewed and approved

[StrikerRUS]

LGTM!

[github-actions]

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