stage2: write out builtin.zig before spawning AstGen tasks

Otherwise it's possible for AstGen to get FileNotFound when trying to
eager-load `import("builtin")`.

src/Module.zig

@@ -109,6 +109,8 @@ stage1_flags: packed struct {
 
 emit_h: ?Compilation.EmitLoc,
 
+job_queued_update_builtin_zig: bool = true,
+
 compile_log_text: ArrayListUnmanaged(u8) = .{},
 
 pub const ErrorInt = u32;