nixos/stage-2: create empty machine-id at boot

Previously, the activation script was responsible for ensuring that
/etc/machine-id exists. However, the only time it could not already
exist is during stage-2-init, not while switching configurations,
because one of the first things systemd does when starting up as PID 1
is to create this file. So I've moved the initialization to
stage-2-init.

Furthermore, since systemd will do the equivalent of
systemd-machine-id-setup if /etc/machine-id doesn't have valid contents,
we don't need to do that ourselves.

We _do_, however, want to ensure that the file at least exists, because
systemd also uses the non-existence of this file to guess that this is a
first-boot situation. In that case, systemd tries to create some
symlinks in /etc/systemd/system according to its presets, which it can't
do because we've already populated /etc according to the current NixOS
configuration.

This is not necessary for any other activation script snippets, so it's
okay to do it after stage-2-init runs the activation script. None of
them declare a dependency on the "systemd" snippet. Also, most of them
only create files or directories in ways that obviously don't need the
machine-id set.

nixos/modules/system/boot/stage-2-init.sh

@@ -152,6 +152,14 @@ ln -sfn /run/booted-system /nix/var/nix/gcroots/booted-system
 @shell@ @postBootCommands@
 
 
+# Ensure systemd doesn't try to populate /etc, by forcing its first-boot
+# heuristic off. It doesn't matter what's in /etc/machine-id for this purpose,
+# and systemd will immediately fill in the file when it starts, so just
+# creating it is enough. This `: >>` pattern avoids forking and avoids changing
+# the mtime if the file already exists.
+: >> /etc/machine-id
+
+
 # Reset the logging file descriptors.
 exec 1>&$logOutFd 2>&$logErrFd
 exec {logOutFd}>&- {logErrFd}>&-

nixos/modules/system/boot/systemd.nix

@@ -787,10 +787,6 @@ in
       ''
         mkdir -m 0755 -p /var/lib/udev
 
-        if ! [ -e /etc/machine-id ]; then
-          ${systemd}/bin/systemd-machine-id-setup
-        fi
-
         # Keep a persistent journal. Note that systemd-tmpfiles will
         # set proper ownership/permissions.
         mkdir -m 0700 -p /var/log/journal