dep: use our fork of golang.org/x/oauth2

Created by: unknwon

Quoted from https://github.com/sourcegraph/oauth2/commit/6a800d6a8d193c79bffb607e9eacb6860bbe0e0c:

GitHub returns 200 status code for invalid client ID and/or client secret, and we have to rely on the response body for error checking.

The error oauth2: server response missing access_token is imprecise and misleading.

See https://docs.github.com/en/free-pro-team@latest/developers/apps/troubleshooting-oauth-app-access-token-request-errors

Tested end-to-end:

Related: https://github.com/sourcegraph/customer/issues/105