Fix the encoding format spec in the liquidsoap template

The right set of parameters depend on the format, move that logic to a
function in the code and out of the template.
3 jobs for v2 in 4 minutes and 20 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build Pkgsrc
passed #7396
build_pkgsrc:stretch

00:00:16

 
  Build Pkg
passed #7397
amd64
build_pkg:stretch:amd64

00:01:20

passed #7398
arm64
build_pkg:stretch:arm64

00:04:03