[ci] switch from MiKTeX to tinytex on Windows R jobs (fixes #5600)

[jameslamb]

Fixes #5600.

R CI jobs require a LaTeX distribution to build the package documentation. This PR proposes switching from MiKTeX to {tinytex} for Windows builds.

Motivation

The use of MiKTeX has been the source of several multi-day outages in LightGBM's CI over the last few years (and most recently, #5600):

• [ci] [R-package] windows-latest CI jobs are broken: "initexmf.exe" failing #5562
• MiKTeX setup error at CI in master #4005
• [R-package] [ci] miktex downloads failing #3454
• [R-package] [ci] R CI jobs failed with to download MiKTeX #3198

Several of those have had the resolution "wait a few days until changes in CTAN mirrors or other upstream sources are fixed". That's very disruptive to our development here, as it means either skipping R package CI jobs or not merging anything for multiple days.

I'm hoping that because {tinytex} is lighter-weight, it'll be more stable.

I also believe using the GitHub Action https://github.com/r-lib/actions/tree/v2-branch/setup-tinytex will remove the maintenance burden of keeping up with MiKTeX and other updates upstream, since that action is actively maintained.

For more, see "Why TinyTex?" in the {tinytex} docs.

[jameslamb]

@guolinke the next time you have a few minutes, can you please review this PR? This will unblock CI, which will unblock #5396, which will prevent CI from breaking starting 8 days from now when Azure removes the macos-10.15 environment.

I'm sorry to keep @-ing you, but there is a lot that needs to get done here, there hasn't been much activity recently from other maintainers, and I am trying to get the project into a good state before some planned travel starting next week.

[guolinke]

@jameslamb no problems, please feel free to @ me when you need any helps.

[jameslamb]

Thank you so much! I'll merge this, merge master into #5396, and then merge #5396 once it builds successfully.

[github-actions]

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