docker-build: CL binary swaps into initramfs after C libs are copied
Moves CL build step between initramfs population (which copies all
shared libs via ldd of C binary) and cpio packing. The CL binary
replaces ac-native in the initramfs, getting all the C binary's libs
(libdrm, libasound, etc.) plus libzstd for SBCL's compressed core.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>