CI: produce new tarballs for the CI in addition to binary tarballs for the download page

[andrewrk]

Right now, build-tarballs CI script creates tarballs & zip files for the download page. As part of this process, it uses zig-bootstrap to compile LLVM, Clang, LLD, and Zig for various targets.

After it finishes doing these things, it would be just one more small extra step to bundle up a different set of tarballs which could be used to update the CI to a new LLVM or zig cc version. This would save some manual labor, usually done by yours truly, every time we need to make these updates.

Instead of uploading these tarballs to ziglang.org/deps/ like I do currently, instead this process could more simply store the most recent ones on the local filesystem, and delete the older ones, capping out at a total of 20 files or something like this. This would save bandwidth costs since we don't really want to update the CI to new tarballs with every commit. The process to update the CI based on these files on the local filesystem would still be manual, but it would not require building LLVM N times on my dev machine.