add minimal conda env option for vcpkg build

[robambalu]

For vcpkg build setups where users may not have root access to the system ( or may not want to update system packages ) it would be useful to have a minimal conda env yml that would be used to test vcpkg builds

[timkpaine]

I think the "minimal env" is just any python we support, followed by pip install csp for the built pypa wheel. That doesn't seem like it should warrant its own yaml as it's one dependency (python, the rest coming from pypa land). Worst case is two if we want graphviz as well.

[robambalu]

This is an env for building csp from source, not testing it. Right now we dont have a good way to do vcpkg builds without sourcing the full conda env which could be misleading / incorrect. Open to other ideas

[timkpaine]

I think it's ok to use the full conda for a vcpkg source build, it's not a "real world" configuration anyway since any usage of conda to build a pypa package is prone to being incorrect (in the sense that you will likely have a non pypa compatible wheel at the end). If we wanted a "this is the minimal amount of conda to be non-conda-ish" it's probably compilers bison flex tar unzip on Mac/Linux.