Write extra_files when building pyembed artifacts v2

[mxk]

These are the changes I needed in order to enable the workflow described in #498 when packaging filesystem-relative files via run-build-script:

1. Write the artifacts directly to OUT_DIR and skip copying them from the build/ directory.
2. Write extra_files to a zstd-compressed tar archive.

Using an archive rather than materializing extra files directly is an improvement on #466, which makes the process faster and simplifies moving the files to their final destination. It also enables the use case where the archive is compiled into the final binary via include_bytes! and then extracted at runtime to always be in the right place. This allows the entire application to still be distributed as a single binary.

I'm intentionally not calling write_extra_files from write_files because the archive is only needed as the output of run-build-script.