feat: expose inner marker_env

[tdejager]

Made the inner public, we used to be able to construct a MarkerEnvironment programatically. However, this functionality has seemed to be removed. As there is now public new method as far as I can see, we cannot use the with_* methods.

We are using this in https://github.com/prefix-dev/pixi/blob/main/src/pypi_marker_env.rs to construct an environment from locked data.

I could also create a new method instead, WDYT?

[konstin]

Would MarkerEnvironment::try_from(MarkerEnvironmentBuilder { ... }) work for you?

[tdejager]

Seems to work! Totally missed that sorry!