AV1 encoder: Avoid holding reference to environment.

The `Environment` reference passed at construction isn't guaranteed to
outlive the call, so make a copy instead. In practice there is a
refcounted instance underneath so copying is cheap.

Bug: None
Change-Id: I621286b927d02a9134d4a734cbb76d6eafb4692a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/474981
Commit-Queue: Erik Språng <sprang@webrtc.org>
Commit-Queue: Sergey Silkin <ssilkin@webrtc.org>
Reviewed-by: Sergey Silkin <ssilkin@webrtc.org>
Auto-Submit: Erik Språng <sprang@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#47789}