centos7 build fails due to missing sphinx

See https://gitlab.haskell.org/ghc/ghc/-/jobs/1048054

Error when running Shake build system:
  at want, called at src/Main.hs:119:44 in main:Main
* Depends on: binary-dist
  at apply1, called at src/Development/Shake/Internal/Rules/Rerun.hs:41:5 in shake-0.19.6-8286668a44703a51ff535fdb488bbcfda41ff46d5b093c47fded2bc0122481dd:Development.Shake.Internal.Rules.Rerun
* Depends on: binary-dist-dir
  at need, called at src/Rules/BinaryDist.hs:228:24 in main:Rules.BinaryDist
* Depends on: docs
  at need, called at src/Rules/Documentation.hs:115:9 in main:Rules.Documentation
* Depends on: _build/docs/users_guide/build-man/ghc.1
  at error, called at src/Builder.hs:445:43 in main:Builder
* Raised the exception:
Non optional builder 'sphinx-build' is not specified in 'hadrian/cfg/system.config' file.
Build failed.

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