Stage wrote PROXMOX_SSH_KEY with printf '%s', so the key file ended
exactly where the secret did. `gh secret set` strips the trailing
newline from a value read from stdin, and OpenSSH refuses a private key
whose last line has none: ssh reports `Load key ...: error in libcrypto`
and the first hypervisor check fails with `Permission denied
(publickey)`. Write it with printf '%s\n'. When the secret kept its own
newline the file ends in a blank line, which OpenSSH accepts.
The contract test now runs Stage with the key both ways and loads the
result with ssh-keygen; against the old step it fails on the first.
Assisted-by: LLM
Signed-off-by: Marian Koreniuk <moriarti@cp.if.ua>