Store the restic snapshot ID in the backup metadata

This will eventually allow us to use --previous and skip the expensive
remote snapshot lookup.
4 jobs for master in 5 minutes and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build Pkgsrc
passed #12326
build_pkgsrc:stretch

00:00:20

 
  Build Pkg
passed #12327
amd64
build_pkg:stretch:amd64

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passed #12328
arm64
build_pkg:stretch:arm64

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  Upload Pkg
passed #12329
upload_pkg

00:00:21