fix(tvix/tracing): Enable tracing in tvix-build.

Change-Id: Ica9734d7774da4d5e2a5cacab6b9feb27becc3e2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12720
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>

tvix/tracing/src/lib.rs

@@ -145,6 +145,7 @@ impl TracingHandle {
     }
 }
 
+#[must_use = "Don't forget to call build() to enable tracing."]
 #[derive(Default)]
 pub struct TracingBuilder {
     progess_bar: bool,