Use frozen snapshots of package repositories

Can get snapshots from https://mran.microsoft.com/documents/rro/reproducibility

````
# from https://resources.rstudio.com/webinars/time-travel-r
R.home()

```bash
cd /opt/R/3.6.0/lib/R
cat etc/Rprofile.site
```

# Add to Rprofile.site 
local({
  print("Setting frozen repo at data snapshot 123")
  options(repos = c(CRAN = "https://...123"))
})

````