Enhancemnet: Certificates can be passed through environment variables

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

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