`project` not validated in POST /api/v1/environments/

[sentry]

Sentry Issue: FLAGSMITH-API-5GR

ValueError: invalid literal for int() with base 10: '<Project ID>'
  File "django/db/models/fields/__init__.py", line 2053, in get_prep_value
    return int(value)

ValueError: Field 'id' expected a number but got '<Project ID>'.
(21 additional frame(s) were not displayed)
...
  File "environments/permissions/permissions.py", line 41, in has_permission
    project = Project.objects.get(project_lookup)