This was done so a default exists, but allows platforms to override as needed

_platform/platform.py: Added CAS call function

_platform/linux.py: Added override to CAS call
_platform/unix.py: Remove CAS call

This is to allow platforms that do not support sandboxing
To error is a controlled/known way

Adding functionality to recognise Darwin as a platform in plaform.py

non-Linux platforms don't return EISDIR when attempting to unlink a directory

Stopped safe_remove attempting to unlink dirs
Previously safe_remove would attempt to unlink a path
Before attempting to remove it if it was a dir

Now it checks for a dir before that step

  - This patch was required due to not being able to execute `bst` on
  a Linux machine without Fuse or Bubblewrap available