Changes to the builder to move towards passing an environment into the
builder [1] can cause some legacy builder types to fail to load [2].
For example:

    Traceback (most recent call last):
      File "/home/runner/work/confluencebuilder/confluencebuilder/.tox/py38-sphinx51/lib/python3.8/site-packages/sphinx/registry.py", line 162, in create_builder
        return self.builders[name](app, env)
    TypeError: __init__() takes 2 positional arguments but 3 were given

The fallback case for preparing deprecated builders should avoid setting
`env`.

[1]: 6ef22d2
[2]: sphinx-contrib/confluencebuilder#691

Signed-off-by: James Knight <james.d.knight@live.com>

Builder implementation would originally always create an optional
environment instance, which an extended builder's implementation could
reference in their own `__init__` call. However, with the change [1] to
support an `env` argument, there is no longer a guarantee that
`self.env` will exist. This commit changes this to restore the creating
of `self.env` if `env` if not passed in.

[1]: 6ef22d2

Signed-off-by: James Knight <james.d.knight@live.com>