Set+handle `PIP_BUILD_CONSTRAINT` the same way as `PIP_CONSTRAINT`

[webknjaz]

The new setting is implemented by pypa/pip#13534 and is available via --build-constraint on CLI. PIP_CONSTRAINT leaking into the underlying ephemeral build env provisioning will stop working next year.

[joerick]

After #2583 we'll leave PIP_CONSTRAINT untouched, so there'll be nothing for us to do here.

Users that want to pin build-system.requires after the pip change might(?) have to switch to using pip as a build frontend in cibuildwheel, unless build provides a feature for this.

[henryiii]

build should just inherit the pip/uv environment variables for this.

[henryiii]

No longer applicable as we no longer mess with PIP_CONSTRAINT either.