Unable to point to ssh keys in ~/.ssh on Jenkins host
This is a reletively recent change, explained in security advisory 440 : https://jenkins.io/security/advisory/2018-06-25/
SECURITY-440 / CVE-2018-1000601 SSH Credentials Plugin allowed the creation of SSH credentials with keys "From a file on Jenkins master". Credentials Binding Plugin 1.13 and newer allows binding SSH credentials to environment variables. In combination, these two features allow users with the permission to configure a job to read arbitrary files on the Jenkins master by creating an SSH credential referencing an arbitrary file on the Jenkins master, and binding it to an environment variable in a job.
SSH Credentials Plugin no longer supports SSH credentials from files on the Jenkins master file system, neither user-specified file paths nor ~/.ssh. Existing SSH credentials of these kinds are migrated to "directly entered" SSH credentials.