tools: make idf_tools.py download --platform argument optional

...as it is described in the docs.

tools/idf_tools.py

@@ -1715,7 +1715,7 @@ def main(argv):  # type: (list[str]) -> None
                          ' It defaults to installing all supported targets.')
 
     download = subparsers.add_parser('download', help='Download the tools into the dist directory')
-    download.add_argument('--platform', help='Platform to download the tools for')
+    download.add_argument('--platform', default=CURRENT_PLATFORM, help='Platform to download the tools for')
     download.add_argument('tools', metavar='TOOL', nargs='*', default=['required'],
                           help='Tools to download. ' +
                           'To download a specific version use <tool_name>@<version> syntax. ' +