[release] Add third-party tarball to release for standalone builds

With the advent of https://reviews.llvm.org/D131919 and
https://github.com/llvm/llvm-project/commit/a11cd0d94ed3cabf0998a0289aead05da94c86eb
 the third-party directory is required to build LLVM and other packages and in standalone
builds the third-party directory is not available from the llvm tarball anymore.

Differential Revision: https://reviews.llvm.org/D137777