Fix koyeb service update when overriding env of a different type

[brmzkw]

Given a service using a plain text environment variable "XXX", then the command koyeb service update app/svc --env XXX=@yyy returns an error because @yyy refers a secret and changing the environment variable type is not supported:

❌ Error while updating the service `xxx/xxx`: the Koyeb API returned an error 400: error_processing_request

🏥 How to solve the issue?
Fix the request, and try again

🕦 The original error was:
400 Bad Request

Output of --debug:

DEBU[0000] ========== HTTP response ==========
HTTP/2.0 400 Bad Request
Content-Length: 85
Content-Type: application/json
Date: Mon, 18 Sep 2023 16:31:56 GMT
Strict-Transport-Security: max-age=15724800; includeSubDomains
X-Ratelimit-Limit: 30, 30;w=60
X-Ratelimit-Remaining: 29
X-Ratelimit-Reset: 4

{"status":400,"code":"error_processing_request","message":"error_processing_request"}

This might be an issue with the API, and not only with the CLI.