cmake: install zig to 'build_dir/stage3' during building
This commit installs Zig to "build_dir/stage3" during building
so that distros' can easily find binary and use it for testing/etc.
This commit also splits "add_custom_target(stage3 ALL" and command that it invokes,
so that it won't retry it during installation,
as target will be considered not out-of-date.
See also https://www.github.com/ziglang/zig/issues/14240#issuecomment-1374642063
339cae7fd0Eric Joldasov committed on 4/30/2023, 11:31:30 PM· committed by Andrew Kelleyparent91b4729