🐞 CI: don't clobber existing .ssh/authorized_keys

We accidentally clobbered the jumpbox's ssh's authorized key, sabotaging
the environment for subsequent builds (usually BATS).

This commit fixes that error by _appending_ rather than clobbering the
`authorized_keys` file.

This error has been causing BATS to fail at ~50% of the time, and
it's been like that since 2023-12-13.

Fixes, when running "bats":

```
Failed to download from 'https://s3.amazonaws.com/bosh-compiled-release-tarballs/bpm-1.2.17-ubuntu-jammy-1.406-20240318-191657-766280616-20240318191658.tgz':
  Unable to download:
    Performing GET request:
      Get "https://s3.amazonaws.com/bosh-compiled-release-tarballs/bpm-1.2.17-ubuntu-jammy-1.406-20240318-191657-766280616-20240318191658.tgz": Creating SOCKS5 dialer: get host key: ssh: handshake failed: ssh: unable to authenticate, attempted methods [none publickey], no supported methods remain
```

ci/tasks/run-lifecycle.sh

@@ -22,7 +22,7 @@ Host jumpbox
         StrictHostKeyChecking no
 EOF
 chmod -R go-rwx ~/.ssh
-sshpass -p $BOSH_VSPHERE_JUMPER_PASSWORD scp ~/.ssh/vcpi.pub jumpbox:.ssh/authorized_keys
+sshpass -p $BOSH_VSPHERE_JUMPER_PASSWORD ssh jumpbox "cat >> .ssh/authorized_keys" < ~/.ssh/vcpi.pub
 
 # grab OpenVPN client configuration & update it with VCPI jumpbox IP
 scp jumpbox:client.ovpn /etc/openvpn/