Add missing line break at end of private key if not present

[fiesh]

This took me quite a while to figure out: Setting a private ssh key without the trailing newline character will result in libcrypto errors when ssh is invoked. (At least using Forgejo, but I can't imagine it's any different on Github.) It would therefore be great if ssh-key-action could check if the supplied private key ends in an \n and if not, append it (or throw a more self-explanatory error or warning).