save sso cache token expiresAt in UTC

If the expiresAt field is saved with time zone (e.g. 2024-06-10T15:00:06-08:00) it will fail to load in certain SDKs such as the rust AWS SDK. To avoid this, ensure that it is always saved as a UTC format.

credentials/ssocreds/sso_cached_token.go

@@ -225,7 +225,7 @@ func (r *rfc3339) UnmarshalJSON(bytes []byte) (err error) {
 }
 
 func (r *rfc3339) MarshalJSON() ([]byte, error) {
-	value := time.Time(*r).Format(time.RFC3339)
+	value := time.Time(*r).UTC().Format(time.RFC3339)
 
 	// Use JSON unmarshal to unescape the quoted value making use of JSON's
 	// quoting rules.