Fixes forward-incompatible behaviour introduced by 0ff08c276f. The idea is to have mod owners migrate from using mod_id.txt to using meson_options.txt, but if prepare.py unconditionally prefers the former, nobody can use the latter.
Removed in 59f89f9a46, brought back to minimize discomfort downstream. prepare.py will still default to the mod_id in meson_options.txt if mod_id.txt is deleted though.
Both the debug and release jobs would upload artifacts, with the same name, which ghactions of course accepted without question. Snapshot 240 happens to have been released correctly (so all binaries are release builds), but that's just luck. I'd thought I'd fixed this problem with another commit, but turns out I hadn't.
Also factor out a bunch of variables so I won't mess up in the future.
Also restructure meson.build and the ghactions workflow a bit, and enable -ffunction-sections and -fdata-sections.
Note that starcatcher uploads have not been tested and most likely don't work.
