Only use -Werror in validate

[alt-romes]

Reverts a change that made -Werror be applied when building in tree Cabal, even when just developing. Puts -Werror back in the cabal.project.validate validation project.

Fixes #9866

[Mikolaj]

@fendor, FYI, the merging process has changed since you made numerous contributions to cabal last time. It's now described in https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#github-pull-request-conventions

Setting the merge label would ensure mergify rebases and runs CI, if needed, so we'd avoid the non-linear history that sneaked in. This also incurs a 2 day delay, which I understand you wanted to avoid, given that devs are inconvenienced by this unintended change continuously. The informal process for avoiding the delay, as people on the Matrix channel would tell you, is to announce the intention in the ticket, ask for objections and wait for a few hours. If there are objections, escalate (e.g., on the Matrix channel), if no objections, merge [edit: rather, set the delay_passed label by hand] and prepare for a discussion and a possible revert later on. If you want to wait even less, announce in the Matrix channel, PM interested parties (that would be @philderbeast in this case), etc. I hope that sounds fair?

[fendor]

I am terribly sorry, this was unintentionally against policy and sincerely apologise. I should have read the conventions more carefully, and will do so before I try to merge a PR next time.

[philderbeast]

I'm good with this change, moving where the -Werror goes.

[Kleidukos]

@Mergifyio backport 3.12

[mergify]

backport 3.12

✅ Backports have been created

• #10022 Only use -Werror in validate (backport #9867) has been created for branch 3.12