Avoid multiple calls to RefreshDetectedYuzuInstallationStateAsync()

[informagico]

Starting EzYuzu with empty Settings.YuzuLocation and selecting the directory via BtnBrowse will result in multiple calls to the "search for updates routine", from both:

private async void CboUpdateChannel_SelectedIndexChangedAsync(object sender, EventArgs e) { ... }
private async void BtnBrowse_ClickAsync(object sender, EventArgs e) { ... }

This change add a guard to the CboUpdateChannel_SelectedIndexChangedAsync event (similar to what has already been implemented in the CboUpdateVersion_SelectedIndexChangedAsync event).

Please consider looking for unexpected behaviours because I'm not 100% familiar with this codebase (yet 😄 ).

[amakvana]

Closed - Merged in v1.6.2.0
https://github.com/amakvana/EzYuzu/releases/latest