buildenv: fix corecrypto image build (archive layout and missing cmake inputs)

[Rogue911]

The buildenv image no longer builds. Apple's public corecrypto archive changed shape,
and two files that its CMake configuration expects are not present in it. Each failure
stops the docker build before the compiler image is produced, so the project cannot be
built from a clean checkout.

Three independent problems, each fixed in buildenv/Dockerfile:

1. The archive no longer extracts to corecrypto/

unzip corecrypto.zip now produces a year-suffixed directory (corecrypto-2024/) plus a
__MACOSX/ metadata directory, so the following WORKDIR /buildenv/corecrypto fails. The
fix removes the metadata directory and normalises the name only when needed, so it keeps
working if Apple reverts to the old layout:

&& rm -rf __MACOSX && (test -d corecrypto || mv corecrypto-*/ corecrypto)

2. scripts/code-coverage.cmake is referenced but not shipped

corecrypto's CMakeLists.txt does include(scripts/code-coverage.cmake). That file is not
part of the archive; it is an external helper from
StableCoder/cmake-scripts, normally added
through a submodule or build step that the public download does not include. The fix
fetches it into scripts/ before configuring.

3. CoreCryptoSources.cmake expects corecrypto_static/ccrng_static.c

The archive ships ccrng_static.c at the top level instead, so configuration fails on a
missing source. The fix symlinks it into the expected path rather than editing Apple's
CMake files, which keeps the change minimal and survives content updates.

Notes

• No source, build flag or output changes — this only makes the dependency image build
  again. The resulting compiler image produces the same binary as before.
• Verified by building buildenv/ from a clean checkout. All three failures occur while
  configuring and building corecrypto, which completes with these fixes and stops before
  them without. The image produced from this Dockerfile then compiles the project using the
  same command the CI workflow uses (make -f ../Makefile -j3 from build/), finishing
  with only the pre-existing -Wreturn-type warnings.