Test compiling stan model in conda with conda-forge compilers installed

[aseyboldt]

Otherwise it seems this can lead to the stan makefile not recognizing the compiler:

RuntimeError: Command make STAN_THREADS=true STANCFLAGS=--include-paths=. /tmp/tmp9av6ksrm/model_model.so failed with code 2.
stdout:

stderr:
stan/lib/stan_math/make/libraries:117: *** "Need to set TBB_CXX_TYPE for non-standard compiler other than gcc or clang.".  Stop.