Install libncursesw5 instead of libncursesw6

DF is hardcoded to open libncurses.so.5 first. Both libncurses.so.5 and
libncurses.so.6 on Ubuntu 20.04 appear to have a valid waddnwstr symbol on my
end, but the GitHub Actions environment might be different.

.github/workflows/build.yml

@@ -30,7 +30,7 @@ jobs:
         sudo apt-get update
         sudo apt-get install \
             libgtk2.0-0 \
-            libncursesw6 \
+            libncursesw5 \
             libsdl-image1.2-dev \
             libsdl-ttf2.0-dev \
             libsdl1.2-dev \