doc-tarball CI job always fails

Recently, the doc-tarball CI job has begun to consistently fail with the following error:

+ cd ..
+ '[' 0 -eq 0 ']'
+ rm -r linux
+ tar -Jxf /builds/ghc/ghc/ghc-x86_64-unknown-mingw32.tar.xz
+ mv ghc-9.1.0.20201016-x86_64-unknown-mingw32 windows
+ cd inst/share/doc/ghc-9.1.0.20201016/html/libraries
+ mv '../../../../../../windows/doc/html/libraries/Win32-*' .
mv: cannot stat '../../../../../../windows/doc/html/libraries/Win32-*': No such file or directory

This happens both for CI jobs on the master branch (example) and on every MR (example).