debian: default to global scopes only, to avoid cardinality explosion

See also ai3/prod#15
4 jobs for master in 3 minutes and 9 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build Pkgsrc
passed #8555
build_pkgsrc:stretch

00:00:16

 
  Build Pkg
passed #8556
amd64
build_pkg:stretch:amd64

00:00:49

passed #8557
arm64
build_pkg:stretch:arm64

00:02:19

 
  Upload Pkg
passed #8558
upload_pkg

00:00:32