cron: fix crontab(5) and cron job PATH

Switch off HAVE_SAVED_UIDS since it activates a code path for temporary
privilege dropping which does not work on NixOS.

Vixie-cron's sources ship with two implementations. Unfortunately, the
one activated by HAVE_SAVED_UIDS (using setuid()) does not work on
NixOS. Saved UIDs work only if the program which is using them has the
setuid bit set on its own executable, not if called from a setuid
wrapper (as we do it in NixOS). The other implementation (using
setreuid()) works without problems.

Quote from
<http://stackoverflow.com/questions/8499296/realuid-saved-uid-effective-uid-whats-going-on>:

    If you're euid is root and you change the uid, the privileges gets
    dropped permanently.If effective user id is not root then saved user
    id is never touched and you can regain the root privilege back
    anytime you want in your program.

Also extend the default PATH with NixOS-specific bin directories as
vixie-cron's default is not really usable on NixOS.

Re #16518

Closes #16522