Uniform buffers are recreated when updating a material asset

## Bevy version

0.8-dev / main (latest: 6a1ba9c456663808aecce2bcdc0a181eed6204d5)

## What you did

1. Define a material with AsBindGroup and Material traits, and with a uniform.
2. Add the material to a mesh through the asset system.
3. Modify the material asset through a system

## What went wrong

The material's bindgroup is recreated every frame (this is fine), however, all the uniform buffers are also recreated.

## Additional information

Relevant Discord discussion: https://discord.com/channels/691052431525675048/1001183621278859324