Using already installed Bun if available

[erikaxel]

It would be nice if setup-bun would be able to detect that a bun binary is already installed, and skip downloading cache or new version if it matches the required version. This way the network would be skipped entirely, which leads to faster runs.

This use case is fairly common when using self hosted runners, since the runners are then typically reused, so bun files will exist from previous runs.

There is some prior art for this in other GitHub tool setups, such as setup-ruby .

Is this something that would be possible?