CI on the `master` branch is broken due to deploy:pages job

The CI on the master branch currently fails due to the deploy:pages job, which consistently fails with the following error (example):

Executing "step_script" stage of the job script
$ mkdir -p public/doc
$ tar -xf haddock.html.tar.xz -C public/doc
tar: haddock.html.tar.xz: Cannot open: No such file or directory
tar: Error is not recoverable: exiting now
ERROR: Job failed: exit code 1

This regression was introduced in c4a69f37 / !4175 (closed). This likely slipped through CI because we do not run the pages:deploy job on MRs, only on master.

This is a problem for head.hackage, which relies on downloading artifacts from the master branch in order to test HEAD. Because the master branch's CI will only produce artifacts for commits that do not produce any CI failures, this means that there have not been any artifacts for over a month. The most recent artifact has since expired, which completely prevents head.hackage from being able to test HEAD. (See here.)

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