Escape newline characters in GitLab variables

Although GitHub allows new line characters in build variables, they should be escaped to \\n. This is because not all ".ini-like" files (e.g. Docker's .env-file) are able to cope with these characters, subsequently making it more difficult to create a GitLab runner like environment. The workaround added here leads to a change in execution (albeit a small one) in each environment.

I think this should just be limited to specifically the ssh key variable rather than a general facility for escaping newlines. AFAIK t ssh key is the only one we have that requires newlines and is the only one for which we are unescaping them.