Oops, set the CI environment variable even earlier.

If I do something like `pytest tests/integration/test_cli.py`, something
about the ordering of imports means that `pipenv.environments` gets
loaded *before* `pytest_sessionstart` runs, which means that
`pipenv.environments.PIPENV_IS_CI` ends up false.

tests/conftest.py

@@ -1,9 +1,14 @@
 import pytest
 import os
 
+# Note that we have to do this *before* `pipenv.environments` gets imported,
+# which is why we're doing it here as a side effect of importing this module.
+# CI=1 is necessary as a workaround for https://github.com/pypa/pipenv/issues/4909
+os.environ['CI'] = '1'
+
 def pytest_sessionstart(session):
-    # CI=1 is necessary as a workaround for https://github.com/pypa/pipenv/issues/4909
-    os.environ['CI'] = '1'
+    import pipenv.environments
+    assert pipenv.environments.PIPENV_IS_CI
 
 
 @pytest.fixture()