A sporadic failure occured on Hydra because a request was sent
to smtpd after the systemd unit was started, but before the daemon
was actually listening. Fix by checking for open ports first.

(cherry picked from commit f70dc57)

This fixes an issue with shells like fish that are not fully POSIX
compliant. The syntax `ENV=val cmd' doesn't work properly in there.

This issue has been addressed in #45932 and #45945, however it has been
recommended to use a single shell (`stdenv.shell' which is either
`bash' or `sh') to significantly reduce the maintenance overload in the
future.

See NixOS/nixpkgs#45897 (comment)

Fixes #45897

/cc @FRidh @xaverdh @etu

(cherry picked from commit df05618)

(cherry picked from commit 0410434)

disable format hardening

(cherry picked from commit e5d9c16)

(cherry picked from commit f89d6d9)