ssh-action@master fails with RSA key: invalid CRT coefficient

When running a GitHub Actions workflow using appleboy/ssh-action@master, the SSH connection fails with the following error:

```
2025/12/02 05:42:09 ssh.ParsePrivateKey: crypto/rsa: invalid CRT coefficient
2025/12/02 05:42:10 ssh: handshake failed: ssh: unable to authenticate, attempted methods [none], no supported methods remain
Error: Process completed with exit code 1.
```

This happened consistently when attempting to connect to an EC2 instance using an RSA private key stored in GitHub Secrets.

## Workflow Configuration

Here is the step that caused the failure:

```yaml

- name: server connect
uses: appleboy/ssh-action@master
with:
host: ${{ secrets.TEST_SERVER_HOST }}
username: ec2-user
key: ${{ secrets.PRIVATE_KEY }}
timeout: 60s
port: 22
```

Once I downgraded to: `appleboy/ssh-action@v1.2.3` the problem was resolved.

Start by reproducing the GitHub Actions workflow with the RSA private key from the issue on appleboy/ssh-action@master, then compare its behavior with v1.2.3. Trace the handling of the private key and the reported invalid CRT coefficient; done means the master version authenticates successfully without requiring a downgrade.

Written by the indexing model from the issue text.