[ci] use wch1/r-debug image in Solaris tests

[jameslamb]

As a result of the changes #3946 , which introduce vignettes/ to the R package (part of #1944), building the R package with R CMD build will include installing the package, since it must be installed to create vignette outputs.

To make this work in the existing Solaris CI job, two changes are required:

• pandoc must be available in the container used to build the package before submitting it to R Hub
• {lightgbm}'s dependencies must be installed before running sh build-cran-package.sh.

This PR proposes both of those changes. It proposes the re-organization of test_r_package_solaris.sh just to make the diff of #3946 slightly smaller and easier to review.

Changing the image used in the Solaris job (to one that already has pandoc installed) needs to be done in a separate PR, since GitHub Actions configs for jobs triggered by comments are sourced from master (#4397 (comment)).

[jameslamb]

just noting this is the same image used in sanitizer jobs:

LightGBM/.github/workflows/r_package.yml

Lines 175 to 179 in da98f24

	test-r-sanitizers:
	name: r-sanitizers (ubuntu-latest, R-devel, ${{ matrix.compiler }} ASAN/UBSAN)
	timeout-minutes: 60
	runs-on: ubuntu-latest
	container: wch1/r-debug

And those jobs are successfully building the package with vignettes.

https://github.com/microsoft/LightGBM/runs/4086356142?check_suite_focus=true

more evidence that this image has pandoc:

docker run --entrypoint="" -it wch1/r-debug pandoc --version

# pandoc 2.5
# Compiled with pandoc-types 1.17.5.4, texmath 0.11.2.2, skylighting 0.7.7
# Default user data directory: /root/.pandoc

[jameslamb]

Solaris job is working with this change! #3946 (comment)

valgrind timed out (#3946 (comment)), tried triggering it again (#3946 (comment))

[github-actions]

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