Drop the UV_LOCK_TIMEOUT=900 CI workaround once cp315 wheels are generally available

Follow-up to https://github.com/reflex-dev/reflex/pull/6930 (Python 3.15 support).

.github/actions/setup_build_env/action.yml now sets UV_LOCK_TIMEOUT: "900" on uv sync, because on the cp315 legs several dependencies (numpy, pandas, scipy, matplotlib, contourpy, ...) have no prebuilt wheels and are built from sdist, which can hold the uv cache lock past the default 300s.

This raises the ceiling for every CI job, not just 3.15, and can mask a genuinely stuck sync by up to 15 minutes.

Once the heavy deps publish cp315 wheels (so the 3.15 legs stop building from sdist):

1. Remove the env: UV_LOCK_TIMEOUT block from setup_build_env/action.yml and its comment.
2. Confirm 3.15 CI times drop back in line with the other versions.