Install libncursesw6

In the past week, run-tests.py has started failing due to an ncurses issue on
Ubuntu 20.04, logged to stdout:

```
Didn't find any flavor of libncursesw, attempting libncurses
Symbol not found: waddnwstr
```

.github/workflows/build.yml

@@ -30,6 +30,7 @@ jobs:
         sudo apt-get update
         sudo apt-get install \
             libgtk2.0-0 \
+            libncursesw6 \
             libsdl-image1.2-dev \
             libsdl-ttf2.0-dev \
             libsdl1.2-dev \