Fix streaming JSON Content-type for postBuild

See #8101

lineDelim is used by streamJSON() so it needs to be set
before its called.

Signed-off-by: Tõnis Tiigi <tonistiigi@gmail.com> (github: tonistiigi)
Merge pull request #8413 from tonistiigi/build-content-type

Fix streaming JSON Content-type for postBuild