Rather than building the Rust parts of libsignal as part of `pod
install`, fetch them from build-artifacts.signal.org. This requires
adding
ENV['LIBSIGNAL_FFI_PREBUILD_CHECKSUM'] = '...'
to the consuming Podfile. The referenced archives are downloaded to
~/Library/Caches/org.signal.libsignal, and are unarchived as part of
the build. (The archives are outside the build directory so that a
clean build does not require a new download.)
Building with LibSignalClient as a local pod is still supported; in
that case everything will refer to the local target/ directory
instead. Use swift/build_ffi.sh to build as usual.
