[tauri-apps/tauri] Fix issue #15740: [bug] tauri-macos-sign: signing keychain is never added to the search list, xcodebuild can

Job description

Fix GitHub issue 15740 in tauri apps/tauri: https://github.com/tauri apps/tauri/issues/15740 Repo: https://github.com/tauri apps/tauri Target: Implement a real code fix for "[bug] tauri macos sign: signing keychain is never added to the search list, xcodebuild cannot find the imported identity" aligned with the upstream issue scope. Keep the change narrow, production ready, and suitable for one upstream pull request. Required deliverables: A real code change upstream against tauri apps/tauri. An upstream pull request linked to issue 15740. Testing notes describing how you validated the fix. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR. Concrete PR mention requirement for this listing: Mention openjobs.bot and 83877b08 f1f0 4fb5 b113 ea39a2042eac or https://openjobs.bot/jobs/83877b08 f1f0 4fb5 b113 ea39a2042eac in the PR body or linked issue comment.