Recipe: find all content that uses a given env var

Possibly to update the value with something else, as in to rotate a token.

Following https://github.com/posit-dev/posit-sdk-py/issues/143, it could be something like:

```python
env_var_name = "MY_SECRET"
new_value = "fa1afe1"

for c in client.content.find():
    if env_var_name in c.env_vars:
        c.set_env_vars({env_var_name: new_value})
```