`libtinfo.so.6` is missing on Ubuntu 18.04

When I try to setup GHC with ghcup on Ubuntu 18.04 LTS, ghc-pkg fails to run because libtinfo.so.6 is missing.

Using deb9 bindist seems to work.

According to https://packages.ubuntu.com/cosmic/libtinfo6, the package libtinfo6, which provides libtinfo.so.6, is only available since Ubuntu 18.10 (cosmic).

Therefore, .download-urls should be changed like this:

ghc 8.6.5   x86_64  debian=9,debian,mint,ubuntu=16.04,ubuntu=18.04  https://downloads.haskell.org/~ghc/8.6.5/ghc-8.6.5-x86_64-deb9-linux.tar.xz

Related issue: #108 (closed)