pre-commit and pre-push only reported total elapsed time at the end,
making it hard to identify which jobs are slow.
Add a `MAKE_TIMED=1` mode that replaces `SHELL` with a wrapper
(`scripts/lib/timed-shell.sh`) to print wall-clock time for each
recipe. pre-commit and pre-push enable this on their sub-makes.
Ad-hoc use: `make MAKE_TIMED=1 test`