feat: add an env variable to toggle pipstar

[aignas]

This is a flag to start leveraging of the new code paths. The Starlark
implementation has been added in 1.4 and has been reverted in the latest
release candidates. The env variable will be a good way to roll it out more
gradually and get more testing.

For now we are switching only the whl_library internals as the
requirements.txt files from uv may use * in python_full_version and
platform_version that are not yet fully supported (#2826).

Main goals for this is to start using Starlark implementation so that we don't
have any hidden variables. What is more, having this in Starlark is the most
maintainable long-term solution for supporting cross-platform builds.

Work towards #260

[rickeylev]

I pushed a fix to wheel_installer_test. CI is passing.

I'll approve so you can merge, but since this is draft, i'll let you trigger merging

[aignas]

I expected this to need more work to pass tests, thanks, will merge now.