Some extensions cache data outside of the `BuildEnvironment` (usually for memory/performance reasons).
For these extensions, it would be helpful to detect when `sphinx-build -E` has been called, which in essence suggests the user wishes to remove cached data before building.
This commit adds the `Sphinx.fresh_env_used` public property, exposing `Sphinx._fresh_env_used` in an immutable manner.
f0dc8239cd