devx: Fix whitespace job output is too verbose

Consider this log: https://github.com/haskell/cabal/actions/runs/12749677510/job/35532656499?pr=10731

I couldn't see immediately where the failure was.

On line 1896, I discover:

```
[ Violation detected ] cabal-testsuite/PackageTests/ExtraProgPathLocal/cabal.project:
cabal-testsuite/PackageTests/ExtraProgPathLocal/cabal.project:3:
cabal-testsuite/PackageTests/ExtraProgPathLocal/cabal.project:4:
```

Either, the failures should be reported at the end, or run in non-verbose mode.

Start with the linked GitHub Actions run and inspect the whitespace job output around the reported violation near line 1896. Trace the job or command that produces this output; done means whitespace failures are immediately visible, either by reporting them at the end or using non-verbose mode.

Written by the indexing model from the issue text.