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_

Beginnen Sie am Einstiegspunkt copilot update und verfolgen Sie die im Bericht gezeigte Download- und Installationssequenz einschließlich der lokalen Paketpfade nach. Reproduzieren Sie eine fehlgeschlagene Installation, gefolgt von einer weiteren Update-Anforderung. Die Aufgabe ist erledigt, wenn ein zuvor heruntergeladenes Paket wiederverwendet wird, anstatt erneut heruntergeladen zu werden, sofern es weiterhin dem angeforderten Update entspricht.