Update primary examples in generate_schema_name doc to also use env_var as well as target.name

### Contributions

- [X] I have read the contribution docs, and understand what's expected of me.

### Link to the page on docs.getdbt.com requiring updates

The [custom schema doc](https://docs.getdbt.com/docs/build/custom-schemas#a-built-in-alternative-pattern-for-generating-schema-names) uses examples that leverages `target.name` over `env_var`. We should add examples that recommend modifying the code to use [environment variables](/docs/build/environment-variables) instead since those can be set at the environment level.

### What part(s) of the page would you like to see updated?

https://docs.getdbt.com/docs/build/custom-schemas#a-built-in-alternative-pattern-for-generating-schema-names

### Additional information

_No response_