Add cached k3s versions to k3s-versions list automatically

[jandubois]

This is mostly relevant to air-gapped setups: it is not enough to put a k3s version in the cache directory; it must also be added to k3s-versions.json. Otherwise the version will be invisible.

This is especially error-prone when setting up an offline config. Rancher Desktop will have a bundled k3s-versions.json, but any newer releases of k3s will not be included.

I think during startup we should add all locally found instances to k3s-versions.json. That makes it also easier to add unreleased k3s versions to the cache for pre-release testing.