SCons: Remove run_in_subprocess & subprocess_main dependencies

[Repiteo]

After getting the idea¹ to check if build-flakiness on Windows is still an issue, it seems that the root cause had everything to do with ensuring that a partially-generated file gets deleted on build failure. For whatever reason, this wasn't consistently happening on Windows, so a pretty massive workaround had to be implemented. This PR implements a new contingency plan in the form of purge_flaky_files: a new atexit function that will ensure any "targeted" files tied to build failures are properly removed. Consequently, the need for the old system's complex workarounds are no longer necessary & the relevant functions have been removed. The only other refactoring necessary was ensuring that the target and source arguments are properly parsed as strings/paths in functions that previously relied on being a subprocess (run_in_subprocess converted those arguments previously, even for non-Windows platforms).

Sidenote: While it's now entirely possible to bring back the generator functions to their respective SCsub files, that's a bit outside this PR's scope. I'd rather that conversion process be part of a general cleanup of the generator functions, as most don't utilize "modern" Python conventions. More importantly, it would risk hiding away the "real" changes of this PR.

Footnotes

1. https://github.com/godotengine/godot/pull/89365#issuecomment-1987345654 ↩

[akien-mga]

Thanks!

[SeleDreams]

Since this got removed, what is the best replacement ? because my 3ds port depended on it to build the final 3dsx

prog = env.add_program("#bin/godot", files)
env.AddPostAction(prog, run_in_subprocess(
    platform_3ds_builders.make_debug_3ds))