can specify chrome tracing file (bevyengine#4618)

# Objective

- I want to have the trace file at a predictable path

## Solution

- Adds an environment variable that is used as the path when it's set

.github/workflows/ci.yml

@@ -283,7 +283,7 @@ jobs:
           for example in .github/example-run/*.ron; do
             example_name=`basename $example .ron`
             echo "running $example_name - "`date`
-            time CI_TESTING_CONFIG=$example xvfb-run cargo run --example $example_name --no-default-features --features "bevy_dynamic_plugin,bevy_gilrs,bevy_gltf,bevy_winit,render,png,hdr,x11,bevy_ci_testing,trace,trace_chrome,bevy_audio,vorbis"
+            time TRACE_CHROME=trace-$example_name.json CI_TESTING_CONFIG=$example xvfb-run cargo run --example $example_name --no-default-features --features "bevy_dynamic_plugin,bevy_gilrs,bevy_gltf,bevy_winit,render,png,hdr,x11,bevy_ci_testing,trace,trace_chrome,bevy_audio,vorbis"
             sleep 10
           done
           zip traces.zip trace*.json

crates/bevy_log/src/lib.rs

@@ -137,7 +137,11 @@ impl Plugin for LogPlugin {
         {
             #[cfg(feature = "tracing-chrome")]
             let chrome_layer = {
-                let (chrome_layer, guard) = tracing_chrome::ChromeLayerBuilder::new()
+                let mut layer = tracing_chrome::ChromeLayerBuilder::new();
+                if let Ok(path) = std::env::var("TRACE_CHROME") {
+                    layer = layer.file(path);
+                }
+                let (chrome_layer, guard) = layer
                     .name_fn(Box::new(|event_or_span| match event_or_span {
                         tracing_chrome::EventOrSpan::Event(event) => event.metadata().name().into(),
                         tracing_chrome::EventOrSpan::Span(span) => {