Hadrian bindist makefile doesn't handle bindists without docs

In https://gitlab.haskell.org/ghc/ghc/-/blob/master/hadrian/bindist/Makefile#L92, install_docs is forcibly added as a dependency of install, even when the build was configured to skip docs entirely. When doing make install from a hadrian bindist sans docs, the install_docs step will fail due to missing files.