By default, the scummvm runner looks up the data directory relative to
the executable. It expects it to be at ../share/scummvm. That works for
a system-installed version of scummvm, where the binary is in /usr/bin
and the data is in /usr/share/scummvm. However, we shouldn't make
assumptions about the directory structure for a custom executable.
This allows setting a custom data directory in addition to the custom
executable.
