GITEA_MIRROR_INTERVAL gets lost after saving settings

[benprisby]

I've noticed that the value I set in the environment variable GITEA_MIRROR_INTERVAL is lost any time I save a setting in the UI, reverting to the default value of 8h.

Steps to reproduce:

1. Set a non-default value in the environment where Gitea Mirror is running, e.g. GITEA_MIRROR_INTERVAL=10m.
2. Sync a repository from the Repositories page in the UI.
3. Verify in Gitea that the value is 10m.
4. Go to the Configuration page and change any value, or simply press a Test Connection button.
5. Go back to the Repositories page and sync a repository.
6. Observe in Gitea that the value is now the default of 8h.

From what I can tell, the only workaround is to restart the Docker container and not touch any settings to ensure the correct interval is respected.