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 Job ID Name Coverage
  Build Src
passed #5694
build:src

00:00:17

 
  Build Pkg
passed #5695
build:pkg

00:01:00

 
  Upload
passed #5696
upload:pkg

00:00:19