[R] brms does not work with R4.0.2

[Witold E Wolski]

Hello,

I upgraded today to R4.0.2 from 3.6.3 on my Windows machine (actually
2 of them - first desktop and then laptop just to verify the problem)
and on both of them I am getting the following error when running the
brms::brm function example code.

```
> bprior1 <- prior(student_t(5,0,10), class = b) +
+     prior(cauchy(0,2), class = sd)
> fit1 <- brm(count ~ zAge + zBase * Trt + (1|patient),
+             data = epilepsy, family = poisson(), prior = bprior1)
Compiling the C++ model
>
> fit1
Error: object 'fit1' not found
```

I tried to get some more information by setting `silent = FALSE` but
nothing is being printed.
All I see is:

```
> brm(count ~ zAge + zBase * Trt + (1|patient),
+     data = epilepsy, family = poisson(), prior = bprior1, silent=FALSE)
Compiling the C++ model
>
```

R tools is installed and I did test that they work properly.
Also:

```
system.file(package = "StanHeaders")
[1] "C:/Users/wewol/OneDrive/Documents/R/win-library/4.0/StanHeaders"
```

Help highly appreciated.

Witek
--
Witold Eryk Wolski