Fix check-style.sh when running using parallel.
Fix check-style.sh when running using parallel.

Parallel launches separate processes and does not grab global variables.
Export `PKG` instead to make sure it gets picked up.

This was causing my `make check` to skip most checks (but still return
success) since all my machines have `parallel` installed.

glycerine/cockroach,vaibhavarora/cockroach,petermattis/cockroach,paperstreet/cockroach,veteranlu/cockroach,petermattis/cockroach,glycerine/cockroach,paperstreet/cockroach,glycerine/cockroach,petermattis/cockroach,cockroachdb/cockroach,glycerine/cockroach,veteranlu/cockroach,cockroachdb/cockroach,glycerine/cockroach,maxlang/cockroach,petermattis/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,petermattis/cockroach,cockroachdb/cockroach,maxlang/cockroach,paperstreet/cockroach,maxlang/cockroach,petermattis/cockroach,veteranlu/cockroach,maxlang/cockroach,petermattis/cockroach,vaibhavarora/cockroach,maxlang/cockroach,cockroachdb/cockroach,veteranlu/cockroach,veteranlu/cockroach,glycerine/cockroach,paperstreet/cockroach,veteranlu/cockroach,cockroachdb/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,cockroachdb/cockroach,veteranlu/cockroach,vaibhavarora/cockroach,maxlang/cockroach,paperstreet/cockroach,vaibhavarora/cockroach,maxlang/cockroach,glycerine/cockroach,vaibhavarora/cockroach,petermattis/cockroach,cockroachdb/cockroach,vaibhavarora/cockroach,paperstreet/cockroach,veteranlu/cockroach,paperstreet/cockroach