Updated flatpak files

[adaell]

1. Created a new flatpak build script that uses the 2024 freedesktop.sdk and the latest version of most of escript's dependencies.
2. Updated flatpak_options.py
3. Created a new file, "flatpak_trilinos.sh", that flatpak-builder uses to build Trilinos.
4. Have fixed a bug in SConstruct that breaks flatpak-builder.

Once these files are in the main repository I will update the build on flathub.

[bztd]

In the manifest, the version of "escript" is not specified. That should generate an error. The current version on Flathub is 5.10. Does version 6.0 use Trilinos as a mandatory dependency?

The current manifest:
https://github.com/flathub/au.edu.uq.esys.escript/blob/master/au.edu.uq.esys.escript.json

[bztd]

I also added pip so I can install additional packages in the sandbox.