Allow term.progress.when to default

[Gordon01]

What does this PR try to resolve?

The config section term.progress contains a collection of options (currently only when and width). If only width is set, Cargo fails with the error: "missing field when." This behavior may be surprising and inconvenient for users who want to set the CARGO_TERM_PROGRESS_WIDTH environment variable.

How should we test and review this PR?

The relevant test, bad_progress_config_missing_when, has been replaced with default_progress_is_auto.

#14615

From #14615 (comment)

I don't recall any particular decisions made regarding bad_progress_config_missing_when. I personally would not object to changing that to allow it to pass. I can't think of a reason it would be required to specify when with width.

[rustbot]

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[weihanglo]

Looks pretty reasonable. Thanks!

[Gordon01]

Amazing!