--filename cli option is currently defined as a global option, but only used by the build command

[rpl]

Is this a feature request or a bug?

bug

What is the current behavior?

The --filename cli option is currently defined as a global option, but the build command is the only one expecting it.

It doesn't make a big difference from a CLI usage perspective, but having it defined as a global option affects its format in the config file, and that may be surprising from a user perspective (e.g. see #2171).

What is the expected or desired behavior?

Move the --filename cli option definition into the build-specific options.

⚠️ This would be a backward incompatible change (from a config file format perspective) and so it would be better to do it as part of a major release and/or include a backward compatible behavior (e.g. warn but still load a config file with a filename global option).