Minor fix to print, set to str (#11934)

* Minor fix to print, set to str

`{commands_filename}` the extra {} actually make it
become a Set, and the output print was like this:
```
Processing json files...
Linking container command to subcommands...
Checking all commands...
Generating {'commands'}.c...
All done, exiting.
```

Introduced in #11920

* more fix