Haddock: use buildSettingKeepTempFiles (backport #9966)

[mergify]

This commit initialises the Haddock flag haddockKeepTempFiles with the value of buildSettingKeepTempFiles.

The only change in the patch is the following, in setupHsHaddockFlags:

- haddockKeepTempFiles = mempty -- TODO: from build settings
+ haddockKeepTempFiles = toFlag keepTmpFiles

where keepTmpFiles is retrieved from BuildTimeSettings.

This addresses an 8 year old TODO in the code.

Template Α: This PR modifies behaviour or interface.

TODO:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated.
• Tests have been added.

---

This is an automatic backport of pull request #9966 done by [Mergify](https://mergify.com).

[mergify]

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Changes to be committed:
	new file:   cabal-testsuite/PackageTests/HaddockKeepsTmps/Simple.hs
	new file:   cabal-testsuite/PackageTests/HaddockKeepsTmps/cabal.project
	new file:   cabal-testsuite/PackageTests/HaddockKeepsTmps/cabal.test.hs
	new file:   cabal-testsuite/PackageTests/HaddockKeepsTmps/my.cabal

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   cabal-install/src/Distribution/Client/ProjectBuilding/UnpackedPackage.hs
	both modified:   cabal-install/src/Distribution/Client/ProjectPlanning.hs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[ulysses4ever]

It's not exactly a bugfix (arguably), and it seems to interact with other Haddock stuff that we decided not to backport, so let's backtrack here and not backport.