Fix profiling with tracy.

The profiling APIs require a `&str`, but since the label here
is now an `Option<String>`, we must get a `&str` from it.

wgpu-core/src/command/render.rs

@@ -1566,7 +1566,7 @@ impl Global {
 
         profiling::scope!(
             "CommandEncoder::run_render_pass {}",
-            base.label.unwrap_or("")
+            base.label.as_deref().unwrap_or("")
         );
 
         let Some(cmd_buf) = pass.parent.as_ref() else {