release.nix: Install the manual under the right prefix.

Dependencies

• [2] JXLJS4RI release.nix: Make the HTML manual a Hydra build product.
• [3] QK2OXCKP hydra/release.nix: move manual to tarball build, restore version in store path for tarball build
• [4] UHMUHQMU hydra: fix tarball build, add pre suffix to tarballs
• [5] Z45QO5BP hydra/release.nix: posthook -> postdist
• [*] T4LLYESZ * Nix expression for building Hydra.

Change contents

• replacement in release.nix at line 32
  [3.20]→[2.0:35](∅→∅)
  − make -C doc/manual install
  [3.20]

  [2.35]
  + make -C doc/manual install prefix="$out"