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

.changes/updater-mobile-install-error.md

@@ -0,0 +1,6 @@
+---
+updater: patch
+updater-js: patch
+---
+
+On Android and iOS, `Update::install` now returns `Error::UnsupportedOs` instead of reporting success without installing anything.