update client to work for v4beta API

[AshleyDumaine]

Currently the regex for parsing the LINODE_URL env var does not work for the v4beta version of the Linode API. This makes it so v4beta can be used if needed (see https://github.com/linode/linodego/blob/main/client.go#L149).

General:

• Have you removed all sensitive information, including but not limited to access keys and passwords?
• Have you checked to ensure there aren't other open or closed Pull Requests for the same bug/feature/question?

Pull Request Guidelines:

1. Does your submission pass tests?
2. Have you added tests?
3. Are you addressing a single feature in this PR?
4. Are your commits atomic, addressing one change per commit?
5. Are you following the conventions of the language?
6. Have you saved your large formatting changes for a different PR, so we can focus on your work?
7. Have you explained your rationale for why this feature is needed?
8. Have you linked your PR to an open issue

[rahulait]

lgtm