Add node to PATH of asterius cabal actions

[ylecornec]

This PR adds node to the tools and PATH of asterius cabal actions.

This is necessary to support Template Haskell and annotations such as the one added in example,
but until now it was only done in the run_ghc case and not the cabal one.

• There is now a path_for_cabal field next to the path_for_runghc field in the tools_config section of the haskell toolchain.
  path_for_cabal does not contain the posix tools, because these are not specific to asterius, and always added to cabal actions.
• These fields are now lists of paths instead of a string, and the join_path_list function is used to join them.

[aherrmann]

Thank you!