Burrito 1.3 now requires Zig 0.15.2 (build fails with 'Your Zig version
does not match the one Burrito requires! We need 0.15.2, you have: 0.13.0').
Zig also changed its tarball naming around 0.15: the arch now comes
before 'linux' (zig-x86_64-linux-VER.tar.xz instead of
zig-linux-x86_64-VER.tar.xz), so both the download URL and the
post-extract symlink glob had to change.
Blocking bootstrap in dev meant you couldn't even run 'mix run' to
generate the initial hash. Now dev/test accept an optional env override
and boot without it; prod still raises when unset.
