Zig's build script has several race conditions preventing proper
concurrent builds from working. By using -j1 for now, finishing this
branch (concurrent zig builds) is untangled from the separate problem of
correcting concurrency issues with zig's own build script.
In other words, let's solve one problem at a time.
Override the cache directories because they won't actually help other CI
runs which will be testing alternate versions of zig, and ultimately
would just fill up space on the hard drive for no reason.
In practice we did see one of the CI servers fill up too many files
inside ~/.cache/zig, which caused certain file system operations to
start returning ENOSPC, despite the hard drive having plenty of space
left.
