Throw an error if GITHUB_TOKEN isn't set

[danielbachhuber]

I keep running into this error when I forget to set GITHUB_TOKEN:

$ wp maintenance release generate db-command
--- wp-cli/db-command ---
Checking milestone '2.0.23'...
Milestone '2.0.23' does not have a matching release, generating one...
Generating the following release:
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Version 2.0.23 (v2.0.23)
- Fix PHP 8.1 fatal error with `--orderby=size` and `--size_format=mb` [[#230](https://github.com/wp-cli/db-command/pull/230)]

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Is the above correct? [y/n] y
Creating release Version 2.0.23 v2.0.23...
Error: Failed request to https://api.github.com/repos/wp-cli/db-command/releases
GitHub API returned: {"message":"Not Found","documentation_url":"https://docs.github.com/rest/reference/repos#create-a-release"} (HTTP code 404)