Build fails with Zig v0.16.0 and higher

While the [README.md](https://github.com/coder/boo/blob/f9aac27cfbc85c409663f1b353ba594e518eed1d/README.md) does clearly state:

> Requires Zig 0.15.2.

it is a bit unexpected that it means a precise version, as opposed to a lower bound of the supported toolchain versions.

Attempting to build on [Alpine](https://alpinelinux.org/) v3.24, which comes with a newer zig, fails with:

> error: Your Zig version v0.16.0 does not meet the required build version of v0.15.2

One could argue the README.md could be phrased more explicitly clear to state that the version requirement is a precise one, but perhaps making the build work with newer versions might be a more appropriate action. It is highly unlikely that any pull-request on this issue will come from me, but I figured it deserved a bug report.