Smaller CI intermediate tarballs (option 1)

[Googulator]

• Move Python bootstrap from pass2 to pass3. Python bootstrap is relatively fast, and we have more than enough time in pass3 to include it there. OTOH, it generates large build artifacts, which needlessly blow up the pass2 tarball size.
• Exclude external/distfiles from stage tarballs. It will always get copied from the top-level distfiles directory, which in turn is reloaded from cache.