Fix library path

We need to set `R_ENVIRON_USER` to an existing file,
otherwise R does not use it. We also need to set R_LIBS
to empty, because otherwise R will pick it up.

Closes #160.

Note that this is also fixed in callr, once
r-lib/callr#84 is merged.
The callr fix is not needed for this revdepcheck fix.

R/check.R

@@ -68,7 +68,8 @@ check_task <- function(state, task) {
 
 check_env_vars <- function(check_version = FALSE, force_suggests = FALSE) {
   c(
-    "R_ENVIRON_USER" = tempfile(),
+    "R_ENVIRON_USER" = tempdir(),
+    "R_LIBS" = "",
     # Switch off expensive check for package version
     # https://github.com/hadley/devtools/issues/1271
     if (getRversion() >= "3.4.0" && as.numeric(R.version[["svn rev"]]) >= 70944) {