bundle: Just use email OID regardless of actual value

Apparently Fulcio does not care about the CSR subject: just claim
everything is an email.

https://github.com/sigstore/fulcio/blob/main/fulcio.proto#L106

Signed-off-by: Jussi Kukkonen <[email protected]>

Author: jku

src/bundle/sign.rs

@@ -85,12 +85,10 @@ impl<'ctx> SigningSession<'ctx> {
         fulcio: &FulcioClient,
         token: &IdentityToken,
     ) -> SigstoreResult<(ecdsa::SigningKey<NistP256>, fulcio::CertificateResponse)> {
+        // NOTE: Currently both email and machine identities get wrapped in a "email" OID.
+        // Fulcio does not care about the content.
         let identity = match &token.identity {
-            Identity::Sub(_) => {
-                return Err(SigstoreError::IdentityTokenError(
-                    "Non-email identities are not yet supported".to_string(),
-                ))
-            }
+            Identity::Sub(identity) |
             Identity::Email(identity) => identity.as_str(),
         };