Return provider-transformed path from variable set

[gergelyurbancsik]

Problem

When invoking confix variable set against the secret provider, the CLI
prints the original input name instead of the encrypted ciphertext, even
though the documentation promises the ciphertext:

$ confix variable set '$secret:irrelevant' 'my-secret-value'
✓ Variable $secret:irrelevant set successfully.    # ← should be $secret:<base64>