nss-systemd,sysusers: make sure sysusers doesn't get confused by nss-systemd (#6812)

In nss-systemd we synthesize user entries for "nobody" and "root", as
fallback if we boot up with an entirely empty /etc. This is supposed to
be a fallback only though, and it's intended that both users exists
regularly in /etc/passwd + /etc/group. Before this patch
systemd-sysusers would never create the entries however as it notices
the synthetic entries. Let's add a way how systemd-sysusers can tell
nss-systemd not to synthesize the entries for itself.

Fixes: #6808

src/sysusers/sysusers.c

@@ -1811,6 +1811,16 @@ int main(int argc, char *argv[]) {
                 }
         }
 
+        /* Let's tell nss-systemd not to synthesize the "root" and "nobody" entries for it, so that our detection
+         * whether the names or UID/GID area already used otherwise doesn't get confused. After all, even though
+         * nss-systemd synthesizes these users/groups, they should still appear in /etc/passwd and /etc/group, as the
+         * synthesizing logic is merely supposed to be fallback for cases where we run with a completely unpopulated
+         * /etc. */
+        if (setenv("SYSTEMD_NSS_BYPASS_SYNTHETIC", "1", 1) < 0) {
+                r = log_error_errno(errno, "Failed to set SYSTEMD_NSS_BYPASS_SYNTHETIC environment variable: %m");
+                goto finish;
+        }
+
         if (!uid_range) {
                 /* Default to default range of 1..SYSTEMD_UID_MAX */
                 r = uid_range_add(&uid_range, &n_uid_range, 1, SYSTEM_UID_MAX);