fix(decision): remove misconfigured environment variable from Decision image

This variable wasn't being used properly. Older versions of uv were
failing silently, but as of uv 0.4.13 this started causing an error.

Author: ahal

taskcluster/docker/decision/Dockerfile

@@ -1,8 +1,6 @@
 FROM $DOCKER_IMAGE_PARENT
 LABEL maintainer="Release Engineering <[email protected]>"
 
-ENV UV_PROJECT_ENVIRONMENT=/setup/taskgraph
-
 # %include src
 # %include packages
 # %include .hatch_build.py