Use github.token for automated GitHub Releases

[victorlin]

Some repos may be using GH_TOKEN_NEXTSTRAIN_BOT_REPO on @nextstrain-bot account for creating GitHub Releases. In a Slack thread with @ivan-aksamentov, we figured out that it's unnecessary and sometimes doesn't work. Instead, the default github.token/GITHUB_TOKEN should be used for this purpose.

[tsibley]

One minor thing: moving from our own token to the default GITHUB_TOKEN will change the release author from "nextstrain-bot" to "github-actions". (You can see this in the Nextstrain CLI releases.) This is fine, but seems worth noting.