SFTP/SCP tests were failing in CI with WinCNG libssh2 since we first
added such job. With curl: (67) Authentication failure.

The reason is that the default ssh-keygen RSA private key format
changed to OpenSSH (RFC4716) in 2018. libssh2 does not support this
format with some of its crypto backends.

Fix it by generating keys explicitly in PEM format as necessary via
the -m option. This format is universally recognized for RSA keys.

2018-08-24: https://www.openssh.com/txt/release-7.8: OpenSSH format becomes default
2010-08-23: https://www.openssh.com/txt/release-5.6: -m option first supported

This fixed the auth issue, just to reveal a known flakiness issue in
libssh2 + WinCNG, causing:

curl: (2) Failure establishing ssh session: -8, Unable to exchange encryption keys

Ref: https://github.com/curl/curl/actions/runs/14000494428/job/39205633258?pr=16781#step:15:1796
Tracked here: libssh2/libssh2#804
Mitigated in libssh2 tests by retrying them.

Due to this, keep ignoring these test results.

Also:

• add an env to customize key format: CURL_TEST_SSH_KEY_FORMAT
• display the generated format in the log.
• GHA/linux: document the wolfSSH error code causing it to fail tests:

  curl: (79) wolfssh SFTP connect error -1051 / WS_MATCH_KEY_ALGO_E / cannot match key algo with peer
  

Follow-up to 4911e7a #16735
Follow-up to 0ec72c1 #16672
Follow-up to e53523f #14859
Follow-up to e26cbe2 #13979