Hi there in case anybody else is having this problem, I opened this GH Issue:
Basically am unable to install cmdstanr using devtools::install_github("stan-dev/cmdstanr")
on a fresh machine image because maybe it tries to compile and run a model upon installation without first checking CmdStan installation / installing / setting the path.
The install using install.packages("cmdstanr", repos = c("https://mc-stan.org/r-packages/", getOption("repos"))
still works.