Fix the "can't keep downloads when asked to" logic

Currently, the warning which is intended to alert users that the download directory can't be kept only appears if they haven't asked for it to be. This change just rearranges the logic a bit so that the warning instead only appears if the download directory *should* be kept, but *can't* be kept (because it doesn't exist).

fixes asdf-vm#1711

lib/functions/installs.bash

@@ -234,15 +234,16 @@ install_tool_version() {
 
     local install_exit_code=$?
     if [ $install_exit_code -eq 0 ] && [ $download_exit_code -eq 0 ]; then
-      # Remove download directory if --keep-download flag or always_keep_download config setting are not set
+      # If the download directory should be kept, but isn't available, warn the user
       always_keep_download=$(get_asdf_config_value "always_keep_download")
-      if [ ! "$keep_download" = "true" ] && [ ! "$always_keep_download" = "yes" ]; then
-        if [ -d "$download_path" ]; then
-          rm -r "$download_path"
-        else
+      if [ "$keep_download" = "true" ] || [ "$always_keep_download" = "yes" ]; then
+        if [ ! -d "$download_path" ]; then
           printf '%s\n' "asdf: Warn: You have configured asdf to preserve downloaded files (with always_keep_download=yes or --keep-download). But" >&2
           printf '%s\n' "asdf: Warn: the current plugin ($plugin_name) does not support that. Downloaded files will not be preserved." >&2
         fi
+      # Otherwise, remove the download directory if it exists
+      elif [ -d "$download_path" ]; then
+        rm -r "$download_path"
       fi
 
       reshim_command "$plugin_name" "$full_version"