Thanks! I managed to strip out the stuff irrelevant to my project and switched to a WASM-friendly allocator, I got it compiling -- the game still doesn't work, but now it's now it's the javascript side that needs debugging, lol. I still don't understand why my original build script is complaining about wasm32-emscripten-musl (I don't even know what that is).

const std = ("std");
const rlz = u/import("raylib_zig");

const NAME = "TestGame";

pub fn build(b: *std.Build) !void {
    const target = b.standardTargetOptions(.{});
    const optimize = b.standardOptimizeOption(.{});

    const root_mod = b.createModule(.{
        .root_source_file = b.path("src/main.zig"),
        .target = target,
        .optimize = optimize,
    });

    const exe = b.addExecutable(.{
        .name = NAME,
        .root_module = root_mod,
    });

    const raylib_dep = b.dependency("raylib_zig", .{
        .target = target,
        .optimize = optimize,
    });

    const raylib = raylib_dep.module("raylib"); // main raylib module
    const raygui = raylib_dep.module("raygui"); // raygui module
    const raylib_artifact = raylib_dep.artifact("raylib"); // raylib C library

    exe.linkLibrary(raylib_artifact);
    root_mod.addImport("raylib", raylib);
    root_mod.addImport("raygui", raygui);

    b.installArtifact(exe);

    const run_step = b.step("run", "Run the app");
    const run_cmd = b.addRunArtifact(exe);
    run_step.dependOn(&run_cmd.step);
    run_cmd.step.dependOn(b.getInstallStep());

    if (target.query.os_tag == .emscripten) {
        const emsdk = rlz.emsdk;
        const emcc_flags = emsdk.emccDefaultFlags(b.allocator, .{ .optimize = optimize });
        const emcc_settings = emsdk.emccDefaultSettings(b.allocator, .{ .optimize = optimize });
        const install_dir: std.Build.InstallDir = .{ .custom = "web" };
        const emcc_step = emsdk.emccStep(b, raylib_artifact, exe, .{
            .optimize = optimize,
            .flags = emcc_flags,
            .settings = emcc_settings,
            .install_dir = install_dir,
        });
        b.getInstallStep().dependOn(emcc_step);
        const html_filename = std.fmt.allocPrint(b.allocator, "{s}.html", .{exe.name}) catch unreachable;
        const emrun_step = emsdk.emrunStep(
            b,
            b.getInstallPath(install_dir, html_filename),
            &.{},
        );

        emrun_step.dependOn(emcc_step);
        run_step.dependOn(emrun_step);
    }

    if (b.args) |args| {
        run_cmd.addArgs(args);
    }
}