[ci] [R-package] re-enable 'rchk' checks

[jameslamb]

Follow-up to #6545

Proposes re-enabling the rchk checks, now that that tool can successfully check lightgbm's source code: kalibera/rchk#35 (comment)

How I tested this

Intentionally caused a stack imbalance by reducing the count in an Rf_UNPROTECT() call by 1: https://github.com/microsoft/LightGBM/pull/6713/files#diff-effa19f4965843414974b8ad1ddcf65710b25c978e203317ce409749841da338

Saw rchk catch it!

Function make_altrepped_raw_vec(void*)
  [PB] has possible protection stack imbalance /__w/LightGBM/LightGBM/lightgbm/src/lightgbm_R.cpp:71
Analyzed 66521 functions, traversed 184251 states.
------------------------------------------------------
rchk found issues

(build link)

Then reverted that, and saw rchk exit normally and not report any issues:

Analyzed 66521 functions, traversed 184251 states.
------------------------------------------------------
rchk did not find any issues

(build link)

[StrikerRUS]

LGTM!
Thank you so much for going to the end with rchk!