Make sure that the generated private key is in PEM format

[mortenpi]

It looks like the default key format generated by ssh-keygen has been changed away from PEM (in OpenSSH 8 it looks like; private key files now start with BEGIN OPENSSH PRIVATE KEY, rather than BEGIN RSA PRIVATE KEY). However, this will be a problem on CI where the OpenSSH is old enough that it doesn't support the new format at all.

So, to make sure that the keys are always compatible with all systems, let's force DocumenterTools to generate them in PEM, which should be lowest common denominator.

X-ref: https://serverfault.com/questions/939909/ssh-keygen-does-not-create-rsa-private-key

[DilumAluthge]

@mortenpi It would be great to get this merged.

[mortenpi]

I was almost going to merge this, but then I realized that older ssh-keygen versions might not support the -m argument. So that should be checked first.

[fredrikekre]

Can use OpenSSH_jll