std.Build: make cache_root and global_cache_root relative to cwd
This makes it so that when there is a tree of std.Build objects, only
one zig-cache is used (the top-level application) instead of polluting
package directories with zig-cache folders.
3f8f63b132Andrew Kelley committed on 2/13/2023, 1:42:26 PMparentd97042a