uv

7efd13ca - Add UV_CONSTRAINT environment variable to provide value for `--constraint` (#3162)

Add UV_CONSTRAINT environment variable to provide value for `--constraint` (#3162) ## Summary This PR is adding `UV_CONSTRAINT` environment variable as analogous to `PIP_CONSTRAINT` to allow providing constraint file via environment variable. Implementing this will simplify adoption of uv in testing procedure in projects that I'm involved (testing using tox). This was my motivation for opening #1841 that is closed in favor of #1789 which was closed without implementing this feature. In this implementation, I have used space as a separator as analogous to `pip`. This introduces an obvious problem if the path contains space. Another option could be to use standard separator (`:` - UNIX like, `;` - Windows). Which one did you prefer? ## Test Plan It is my first contribution and first rust coding experience. It will be nice if one could point how I should implement testing this.