[ci] [R-package] run 'R CMD check' as a foreground task

[jameslamb]

Contributes to #6498

4 years ago, this project was running a lot of its R-package testing on Travis CI. We experienced an issue where R CMD check would run for too long without emitting a new log line, and as a result Travis killed jobs.

A workaround was introduced in #3092 ... running R CMD check as a background task and polling for the status of it's process ID.

I strongly suspect this is no longer necessary:

• we are not on Travis any more (and I suspect that GitHub Actions does not have a similar timeout based on log output)
• the R package compiles faster in CI than it did 4 years ago

This proposes removing that workaround, to hopefully make it a bit easier to understand the CI setup. It also fixes some shellcheck errors, mentioned in #6501 (comment)

How I tested this

Checked a few of the logs to be sure R CMD check is really being run and is succeeding. Looks good to me!

https://github.com/microsoft/LightGBM/actions/runs/9788164564/job/27025779285?pr=6508#step:9:2015

If it wasn't safe to remove this, we'd see jobs outright fail. I'm pretty confident this is safe to do.

[github-actions]

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