key file is not used if other keys exist in ssh agent

While debugging a `k0sctl` issue, I found my specified `keyPath` for ssh is not used and ssh connection failed.
```yaml
spec:
hosts:
- ssh:
address: rocky9-0
keyPath: /home/docker/.ssh/id_ed25519
port: 22
user: root
...
```
To work around the issue, I had to manually add the key to ssh agent or unset `SSH_AUTH_SOCK` or clearing keys in ssh agent.

I reviewed `k0sproject/rig` codes and found the reason is that I have already had other ssh keys in SSH agent, and the specified ssh key file is configured as another `ssh.AuthMethod` in https://github.com/k0sproject/rig/blob/v0.19.0/ssh.go#L434 , so `ssh.ClientConfig.Auth` has two `AuthMethod`, both `method(`) is `publickey` , `golang.org/x/crypto/ssh client_auth.go` only tries the first one for the same auth method (https://cs.opensource.google/go/x/crypto/+/refs/tags/v0.29.0:ssh/client_auth.go;l=101), so the specified ssh key file is never used.

Start in ssh.go around line 434, where the SSH authentication methods are assembled, and compare that behavior with golang.org/x/crypto/ssh client_auth.go. Reproduce the case with keys already present in SSH_AUTH_SOCK and a separate keyPath. Done means the configured key file is tried successfully even when other public-key methods are available.

Written by the indexing model from the issue text.