[scripts] have build-zircon and make-parallel exec the subjob

Since they're the last thing it was running anyway, may as well exec it.

Change-Id: I05da1948b1a20068cbe1c5cc512bfb039262f098
diff --git a/scripts/build-zircon b/scripts/build-zircon
index 62599d5..7766176 100755
--- a/scripts/build-zircon
+++ b/scripts/build-zircon
@@ -50,4 +50,4 @@
     HELP
 fi
 
-${DIR}/make-parallel ${ARCH} ${ARGS} "$@"
+exec ${DIR}/make-parallel ${ARCH} ${ARGS} "$@"
diff --git a/scripts/make-parallel b/scripts/make-parallel
index 621bc0f..a7fe8f4 100755
--- a/scripts/make-parallel
+++ b/scripts/make-parallel
@@ -25,4 +25,4 @@
     ;;
 esac
 
-$MAKE $PARALLEL "$@"
+exec $MAKE $PARALLEL "$@"