Extraction of symlinks must be deferred

[gaborcsardi]

Or maybe we can create broken symlinks.

Error in zip::unzip("/tmp/BPCells.zip") :
  Cannot create symlink `bnprks-BPCells-0162964/cpp/bpcells-cpp` in archive `/private/tmp/BPCells.zip` @rzip.c:198 (R_zip_error_handler)

Cf. r-lib/pak#807 (comment)