contrib/actions: drop version pin on libncurses-dev

Fixes unmet dependencies:

 The following packages have unmet dependencies:
 libncurses5-dev : Depends: libtinfo5 (= 6.1-1ubuntu1.18.04) but 6.2-0ubuntu2 is to be installed
                   Depends: libncurses5 (= 6.1-1ubuntu1.18.04) but 6.2-0ubuntu2 is to be installed
                   Depends: libtinfo-dev (= 6.1-1ubuntu1.18.04)
 python : PreDepends: python-minimal (= 2.7.15~rc1-1) but it is not going to be installed
          Depends: libpython-stdlib (= 2.7.15~rc1-1) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.

Switch to apt-get, because it has a stable API. Same as ca7a8ff5.