unified/build_unified: Use build/<mode>/dist/tar for dependency tarballs
unified/build_unified: Use build/<mode>/dist/tar for dependency tarballs

The build_unified.sh script has the same bug as configure.py had: it
looks for the python tarball in
build/<mode>/scylla-python3-package.tar.gz, but it's never generated
there. Fix up the problem by using build/<mode>/dist/tar location for
all dependency tarballs.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla