Enhancement: Allow certificate paths to be passed through environment variables

Restic will now read paths to certificates from the environment variables
`RESTIC_CACERT` or `RESTIC_TLS_CLIENT_CERT` if `--cacert` or `--tls-client-cert`
are not specified. 

https://github.com/restic/restic/issues/1926  
https://github.com/restic/restic/pull/4384
