Overwrite files if they exist

The default behavior of `file.copy()` is _not_ to overwrite files
(without an error or warning) if they already exist. So if there was
already a built tarball in the check directory it would not be updated.

cc @jennybc

R/build.R

@@ -28,7 +28,7 @@ build_package <- function(path, tmpdir, build_args, libpath, quiet) {
     )
 
   } else {
-    file.copy(path, tmpdir)
+    file.copy(path, tmpdir, overwrite = TRUE)
     file.path(tmpdir, basename(path))
   }
 }

inst/NEWS.md

@@ -1,6 +1,9 @@
 
 # devel
 
+* `rcmdcheck()` now correctly overwrites existing tarballs if they already
+  exist in the check directory (#84 @jimhester).
+
 # 1.3.0
 
 * New `rcmdcheck_process` class to run `R CMD check` in the background.