Work around a bug in R 4.1

R 4.1 has an obscure bug in `get0('.First')`, which causes the function
to return `NULL` instead of `ifnotfound`, unless `mode` is specified,
because `.First` is defined as `NULL` in the ‘base’ package:

```
> get0('.First', ifnotfound = 42)
NULL
> get0('.First', mode = 'function', ifnotfound = 42)
[1] 42
```

tests/testthat/support/rprofile.r

@@ -23,7 +23,7 @@ local({
         sys.source(user_profile, envir = .GlobalEnv)
     }
 
-    old_first = get0('.First', envir = .GlobalEnv, ifnotfound = function () {})
+    old_first = get0('.First', envir = .GlobalEnv, mode = 'function', ifnotfound = function () {})
     .GlobalEnv$.First = function () {
         old_first()
         # This is required by `interactive_r` to verify that invocation succeeded.