collect artifacts

[mcandre]

Collect the individual binaries into a sparse directory tree, that holds only debug or release executables, and no other files.

This will make it easier to subsequently zip or tarball the binaries into an archive file for uploading.

tgz format is good to use, however, we should also take pains to apply chmod +x bits to the relevant paths (basically, all UNIX artifact names lacking a file extension). In case the user is using an uncooperative host file system that may accidentally drop chmod bits. (Basically, non-WSL Windows developers.)