Use . as separator for filename when streaming to restic

Restic 0.9.3 fails when --stdin-filename contains slashes:

repository 954a4e61 opened successfully, password is correct
error: file does not exist
Fatal: unable to save snapshot: snapshot is empty
3 jobs for master in 1 minute and 37 seconds
Status Name Job ID Coverage
  Build Src
passed build:src #5694

00:00:17

 
  Build Pkg
passed build:pkg #5695

00:01:00

 
  Upload
passed upload:pkg #5696

00:00:19