appimage: Use a different default for the AppImage build directory

* build.sh: Set builddir if unset after parsing the command line.

Use ~/b/SRCDIRNAME-appimage instead of ~/b/SRCDIRNAME-playground as
default build directory for the AppImage. Using the same build directory
for gpg4win and the AppImage will certainly end in chaos.

• GnuPG-bug-id: T7509