sharp edge: CODER_OIDC_EMAIL_DOMAIN does not work if it contains an '@' symbol

If `CODER_OIDC_EMAIL_DOMAIN` is configured with an '@' symbol (e.g. `CODER_OIDC_EMAIL_DOMAIN=@example.com`), then OIDC logins will fail with the message

> `Your email "user@example.com" is not in domains ["@example.com"]!

Potentially related to https://github.com/coder/coder/commit/4439a920e454a82565e445e4376c669e3b89591c 

Workaround: remove the '@' symbol.