Replace deeplink exp scheme handling with an environment variable

[rolznz]

We added some code to test deeplinks with expo's recommended method, but it's actually quite limited and doesn't allow passing query parameters with a custom host (how we handle deeplinks from push notifications), so I think we should remove that code and instead add a very simple check to overwrite the current link with an env variable if it's set.

• https://github.com/getAlby/go/blob/master/lib/link.ts#L22
• https://github.com/getAlby/go/blob/master/lib/link.ts#L121-L133
• https://github.com/getAlby/go/pull/369/files#r2501862189 <-- should be loaded from an environment variable instead