Clear the install marker on elevation failures too for PR #7490

The code 2 exit keeps the marker for the elevated run that follows, but every
terminal failure of that run, a denied prompt or a failed apt-get included,
returned without clearing it. That pinned a working install into repair the
same way a failed update did. finish_elevation_failure is the one path all of
them take.

studio/src-tauri/src/install.rs

@@ -914,6 +914,9 @@ fn finish_elevation_failure(
     exit_status: Option<String>,
     message: String,
 ) {
+    // Terminal, like a cancelled prompt: the run the code 2 exit left the
+    // marker for is not happening.
+    clear_install_marker_best_effort();
     if let Some(attempt) = attempt {
         diagnostics::finish_attempt(
             diagnostics,