fix(updater): report install as unsupported on mobile (#3695)

`#[cfg(mobile)] fn install_inner` returned `Ok(())` without doing anything.
`check` already fails with `UnsupportedOs` there unless a Rust caller sets a
custom target, in which case a download was followed by a successful
"install" that did nothing. It now returns `Error::UnsupportedOs`, the error
`check` uses.

Verified with clippy for aarch64-linux-android (the iOS build needs the Swift
package, which does not build here).

plugins/updater/src/updater.rs

plugins/updater/src/updater.rs

@@ -928,9 +928,10 @@ impl Update {
         self.install_inner(&bytes)
     }
 
+    /// Updates are not supported on mobile, where apps are updated through the app stores.
     #[cfg(mobile)]
     fn install_inner(&self, _bytes: &[u8]) -> Result<()> {
-        Ok(())
+        Err(Error::UnsupportedOs)
     }
 
     /// Whether the Windows installer should restart the app after installed, default is `true`