Makefile.inc1: show time for `make installworld'

For years we display the time in seconds how long it takes to
run `make buildworld' (see PR 224433). Now we will display the
time for "installworld" and "installkernel" as well.

e.g.:

Installing everything completed on Sun Jul  7 16:11:37 UTC 2024
Install world completed in 110 seconds, ncpu: 2, make -j2

This is an improved version of commit e5a0202f96948a95bf1b879727e155a4f1d1da93

PR: 280187
Differential Revision: https://reviews.freebsd.org/D45912