Allow using environment variables to set basic defaults for repos

While looking at the JAX project, a somewhat convenient feature they have is setting an environment variable to override the URL and sha that is used.

This does seem like a pretty convenient and low-overhead way to quickly switch up the URL used if you're playing around with the toolchain. Much easier than figuring out the `python.single_version_platform_override()` apis. As an example, it would have made it easier to test the new python-build-standalone builds that changes the linking of tk/tcl libs.

Start with the python.single_version_platform_override() APIs and the JAX behavior described in the issue. Determine how environment variables should override the repository URL and sha, including their names and precedence; done means users can switch those defaults without using the APIs.

Written by the indexing model from the issue text.