haddock-project fixes (backport #8919) #3294

Job	Run time
Validate ${{ matrix.os }} ghc-prerelease	0s
Validate ubuntu-20.04 ghc-9.4.5	9m 17s
Validate ubuntu-20.04 ghc-9.2.8	9m 17s
Validate ubuntu-20.04 ghc-9.0.2	9m 5s
Validate ubuntu-20.04 ghc-8.10.7	9m 19s
Validate ubuntu-20.04 ghc-8.8.4	9m 19s
Validate ubuntu-20.04 ghc-8.6.5	9m 20s
Validate ubuntu-20.04 ghc-8.4.4	9m 16s
Validate macos-latest ghc-9.4.5	9m 8s
Validate macos-latest ghc-9.2.8	9m 14s
Validate macos-latest ghc-9.0.2	9m 14s
Validate macos-latest ghc-8.10.7	9m 3s
Validate macos-latest ghc-8.8.4	9m 13s
Validate macos-latest ghc-8.6.5	9m 15s
Validate macos-latest ghc-8.4.4	9m 14s
Validate windows-latest ghc-9.4.5	9m 12s
Validate windows-latest ghc-9.2.8	9m 12s
Validate windows-latest ghc-9.0.2	9m 11s
Dogfooding ${{ matrix.os }} ghc-${{ matrix.ghc }}	1s
Validate old ghcs ${{ matrix.extra-ghc }}	1s
Validate post job	2s
	2h 36m 53s

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