`cabal-validate`: if `stdout` isn't a tty, use GitHub Actions grouping

Small suggestion: if `stdout` isn't a tty, maybe output [GitHub Actions grouping](https://docs.github.com/en/actions/writing-workflows/choosing-what-your-workflow-does/workflow-commands-for-github-actions#grouping-log-lines) that can be collapsed or expanded by the viewer instead of the ANSI escapes. Then we can make everything verbose in CI without overwhelming people trying to go through the log.

_Originally posted by @geekosaur in https://github.com/haskell/cabal/issues/10569#issuecomment-2492005062_

Locate the cabal-validate output handling and inspect how ANSI escapes are emitted for grouped or verbose output. Exercise the non-TTY path in GitHub Actions and verify that log sections use collapsible grouping without ANSI escapes.

Written by the indexing model from the issue text.