The default is that the files are generated in /tmp/Generatedinterfaces on the host.
This path is not accessible from inside the flatpak sandbox.
This is basically for API descriptions for Foundation or other libraries. swift has some API description stored in a binary format (Remotely comparable to typelib). If you want to go to the definition of a symbol, it generated a .swiftinterface file with the API declarations and docs (Think of something comparable to a .vapi file). This commit fixes this by changing an option to generate those files not in /tmp/GeneratedInterfaces, but in .build/GeneratedInterfaces (.build is the default build directory for the swift package manager, so it shouldn't collide with anything, I hope)