new Flatpak binaries for nightly build

[Ben Pfaff]

Today, I started building Flatpak binaries as part of the nightly build.
If you have a system with Flatpak available (which includes many GNU/Linux
systems), you can install the latest PSPP by directing your web browser to:
https://benpfaff.org/~blp/pspp-master/latest-org.fsf.pspp.flatpakref

Please do try it and let me know how it goes. It seems to work OK for me
on the first try.

After the upcoming release, I'll try to get a stable release of PSPP into
flathub so that it's not necessary to use a nightly build.

The manifest and flatpakref templates I use for the build are here:
https://pintos-os.org/cgi-bin/gitweb.cgi?p=pspp;a=shortlog;h=refs/heads/build