Make sure to install to a clean prefix

* Otherwise, existing files in the prefix could easily conflict and
  potentially cause very hard to debug errors.
* This is particularly important for *-dev builds which might be
  repeatedly installed to the same directory.

bin/ruby-build

@@ -756,6 +756,8 @@ remove_windows_files() {
 }
 
 build_package_copy() {
+  # Make sure there are no leftover files in $PREFIX_PATH
+  rm -rf "$PREFIX_PATH"
   mkdir -p "$PREFIX_PATH"
   cp -fR . "$PREFIX_PATH"
 }