avoid redownload if the previous update fails

Describe the feature or problem you'd like to solve

when copilot is requested to update itself, it downloads a package and then installs it. if the installation fails for whatever reason, the next update request downloads the package once again without trying to reuse the previously downloaded update

Proposed solution

Here is a shell snippet indicating the fact that Copilot tries to redownload the update package when the previous attempt to update failed

Caching binaries in the local filesystem can avoid this and isn't a bad idea until the next update arrives

~/git-repos$ copilot update
Checking for updates...
Checking GitHub for the latest release...
Update available: v1.0.84-3 (current: 1.0.84-1).
Downloading update package...
Download complete. Installing...
Error during update: Failed to download package: Error: EPERM: operation not permitted, rename 'C:\Users\myself\.copilot\pkg\tmp\1.0.84-3-13492-1789030021787' -> 'C:\Users\myself\.copilot\pkg\win32-x64\1.0.84-3'

~/git-repos$ copilot update
Checking for updates...
Checking GitHub for the latest release...
Update available: v1.0.84-3 (current: 1.0.84-1).
Downloading update package...
Download complete. Installing...
Copilot CLI version 1.0.84-3 installed.

### Example prompts or workflows

_No response_

### Additional context

_No response_

Parti dal punto di ingresso copilot update e segui la sequenza di download e installazione mostrata nel report, inclusi i percorsi dei pacchetti locali. Riproduci un’installazione non riuscita seguita da un’altra richiesta di aggiornamento. Il lavoro è completato quando un pacchetto scaricato in precedenza viene riutilizzato invece di essere scaricato di nuovo, se corrisponde ancora all’aggiornamento richiesto.