Add Conda based env kits

This PR adds support for getting env kits based on conda environments for both the stable and unstable flavours, in addition to the existing LCG-based ones.

I decided to edit the platform string to match the "subdir" string that conda uses to define the platform (e.g. linux-64, osx-64 or linux-aarch64). This makes it easy to create and install the new kits along side the old ones and we can edit the activation scripts when we're ready.

We can also create environments for out-dated platforms by creating modified versions such as linux-64@cos5 based on a frozen environment.yml file.

Support for building environments for other platforms like ARM or POWER is mostly here but the use of pip inside the environment.yml file causes this to fail (as python -m pip can't be executed). It can wait until a later PR but the simplest solution is probably to create a conda package for everything so there is no need to execute code at install time.