fix: repos.override() to handle multiple env-var named repos

[JosiahParry]

Following #2200 the renv::install() and renv::restore() failed to properly resolve the repos from the environment variable RENV_CONFIG_REPOS_OVERRIDE.

This pr closes #2209 (sorry!) by calling renv_bootstrap_repos() in renv_config_decode_envvar() when the envname is RENV_CONFIG_REPOS_OVERRIDE

I've added an install test as well to make sure this is tested properly!

[JosiahParry]

Looks like CI is failing to install data.table on MacOS

clang -arch arm64 -std=gnu2x -I"/Library/Frameworks/R.framework/Resources/include" -DNDEBUG   -I/opt/R/arm64/include    -fPIC  -falign-functions=64 -Wall -g -O2  -c assign.c -o assign.o
In file included from assign.c:1:
In file included from ./data.table.h:25:
./po.h:2:10: fatal error: 'libintl.h' file not found
    2 | #include <libintl.h>
      |          ^~~~~~~~~~~
1 error generated.

unsure how I should address this :o

[remlapmot]

I believe you need LLVM 16

brew install llvm@16

withr::with_makevars(
  c(CC = "/opt/homebrew/opt/llvm@16/bin/clang"),
  install.packages("data.table", type = "source", repos = c(CRAN = "https://cloud.r-project.org/"))
)

[JosiahParry]

Thanks @remlapmot! I suspect that we ought not need to change the CI runner / script though for this PR?