In #1897 we replaced a 'wc -l' with a bash-native job counter, but
subsequently discovered that bash on MacOS folds '<<<' output into
a single line, preventing line counting.
A for loop works around that problem, is still bash-native, and works
on Linux as well.
While we're at it, also removed the need for command substitution and
an echo by doing the work directly on NUM_JOBS.
Fixes #2241.
3127a9aa87