Manually parse and reconcile Boolean environment variables (#17321)
## Summary
This gives us more flexibility since we can avoid erroring on
"conflicts" when one option is disabled (e.g., `UV_FROZEN=0 uv lock
--check`).
Closes https://github.com/astral-sh/uv/issues/13385.
Closes https://github.com/astral-sh/uv/issues/13316.
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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
