[R-package] fix learning-to-rank tests on Solaris

[jameslamb]

This is a (hopefully temporary) workaround for #3513, as suggested in #3513 (comment). See that issue for details.

I think this is necessary for the 3.1.0 release (#3484 ), because without it the R package might fail CRAN's checks on Solaris.

I have tested on R Hub and confirmed this allows the R package to pass R CMD check on Solaris:

sh build-cran-package.sh

result <- rhub::check(
    path = "lightgbm_3.0.0.99.tar.gz"
    , email = "jaylamb20@gmail.com"
    , check_args = c(
        "--as-cran"
    )
    , platform = c(
        "solaris-x86-patched"
        , "solaris-x86-patched-ods"
    )
    , env_vars = c(
        "R_COMPILE_AND_INSTALL_PACKAGES" = "always"
        , "_R_CHECK_FORCE_SUGGESTS_" = "true"
        , "_R_CHECK_CRAN_INCOMING_USE_ASPELL_" = "true"
    )
)

[StrikerRUS]

LGTM!

[github-actions]

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