gitlab-ci: Don't build PDF user's guide on AArch64
gitlab-ci: Don't build PDF user's guide on AArch64

For reasons I don't understand sphinx seems to fail to produce a .idx
file for makeindex.

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc