Add ability to augment cross environment PATH configurations #101

What is the problem or limitation you are having?

On iOS, the value of PATH is explicitly cleared to ensure there's no leakage of host platform libraries into built wheels. However, these are tools that will sometimes be needed, but can't be included in the "clean" path.

xbuild should have an ability to augment PATH, in addition to overwriting it.

Describe the solution you'd like

If the environment defines an XBUILD_PATH environment variable, that variable should be prepended to the PATH variable used when a cross-platform environment is defined.

This should be honoured on all platforms, even though it's really only needed for iOS.

Describe alternatives you've considered

Require the use to put binaries into a safe path (e.g., the venv's bin directory). This isn't ideal because we can't guarantee what is already in that path.

Additional context

No response