Fix misleading error message when using an expired GITHUB_TOKEN

[jay-aye-see-kay]

Summary

Currently when if GITHUB_TOKEN is set to an expired or invalid token requests to github will fail with a 404. There is not messaging to the user (even with DEVBOX_DEBUG=1) to indicate the cause. This took a while to debug, opening this PR to make it easier for the next person this happens to.

How was it tested?

• make a temp devbox project and add a plugin like https://github.com/jetify-com/devbox-plugins/tree/main/mongodb
• set GITHUB_TOKEN to a random string
• see the following error that doesn't indicate the cause of the error at all (also try running with DEVBOX_DEBUG=1 and see no extra helpful info

❯ devbox install

Error: failed to get plugin github:jetpack-io/devbox-plugins?dir=mongodb @ https://raw.githubusercontent.com/jetpack-io/devbox-plugins/master/mongodb/plugin.json (Status code 404). 
Please make sure a plugin.json file exists in plugin directory.

• now using code in this PR

❯ ../dist/devbox install

Error: failed to get plugin github:jetpack-io/devbox-plugins?dir=mongodb @ https://raw.githubusercontent.com/jetpack-io/devbox-plugins/master/mongodb/plugin.json (Status code 404).
The auth header `token gh_1*********` was send with this request.
Please make sure a plugin.json file exists in plugin directory.

Community Contribution License

All community contributions in this pull request are licensed to the project
maintainers under the terms of the
Apache 2 License.

By creating this pull request, I represent that I have the right to license the
contributions to the project maintainers under the Apache 2 License as stated in
the
Community Contribution License.

[savil]

LGTM, thanks for sending this!

Will let cicd tests run and pass prior to landing