Handle remote prune failures gracefully on self-hosted runners

[peter-evans]

Towards #4198

Wrap the git remote prune command in a try-catch block to prevent the action from failing if the prune operation fails. Instead, log a warning message and allow the action to continue.

Fixes edge cases where the prune command may fail on self-hosted runners but shouldn't block the pull request creation workflow.

[github-actions]

Full test suite slash command (repository admin only)

/test repository=peter-evans/create-pull-request ref=catch_failed_prune build=true

/test repository=peter-evans/create-pull-request ref=catch_failed_prune build=true sign-commits=true