rchk: fix 'R CMD config' parsing

Fixes #96 Since https://github.com/wch/r-source/commit/cf588ba6151d4c6cbecf5b4b9e15e6302b1d53f8, on R-devel `R CMD config --all` now ends like this: ```text ## The following variables are defunct CPP CXXCPP CXX11 CXX11STD CXX11FLAGS CXX11PICFLAGS CXX14 CXX14STD CXX14FLAGS CXX14PICFLAGS SHLIB_CXX11LD SHLIB_CXX11LDFLAGS SHLIB_CXX14LD SHLIB_CXX14LDFLAGS ``` This PR modifies a script here that parses that output, to ignore that line and therefore avoid creating a malformed Makevars. ## Notes for Reviewers ### How I tested this Tried with a package that has C and C++ code. ```shell docker run \ --rm \ --platform linux/amd64 \ -it ghcr.io/r-hub/containers/rchk:latest \ bash mkdir -p /tmp/packages cd /tmp/packages curl -O https://cran.r-project.org/src/contrib/lightgbm_4.6.0.tar.gz Rscript -e "install.packages(c('R6', 'jsonlite', 'data.table'), repos = 'https://cran.r-project.org')" r-check $(pwd) ``` On the latest image pulled from GHCR, that fails in the way described on #96. Manually applying this patch to `/opt/R/devel-rchk/bin/rchk.sh`, it succeeds. Thanks for your time and consideration.