demo/data/meson.build: Don't require `sh` to be available on Windows

Hi,

From the commit message:

The sh program may not be available if one is building libadwaita on Windows with Visual Studio or clang-cl, so:

• Only look for it if the date program is found--note that the built-in one that is provided with cmd.exe shells used for Visual Studio and clang-cl builds cannot be found or run by Meson.
• Make finding the sh program optional on Windows, since it is not available by default on Windows, after the date program can be found.

Notice that the date command that is provided by Windows cannot be found using Meson's find_program(), and it does not do what we want here.

With blessings, thank you!