nixos/atd: prefer 'install' over 'mkdir/chmod/chown'

I don't think there was a security issue here, but using 'install' is
preferred.

Ref #121293.

nixos/modules/services/scheduling/atd.nix

@@ -81,14 +81,9 @@ in
         jobdir=/var/spool/atjobs
         etcdir=/etc/at
 
-        for dir in "$spooldir" "$jobdir" "$etcdir"; do
-          if [ ! -d "$dir" ]; then
-              mkdir -p "$dir"
-              chown atd:atd "$dir"
-          fi
-        done
-        chmod 1770 "$spooldir" "$jobdir"
-        ${if cfg.allowEveryone then ''chmod a+rwxt "$spooldir" "$jobdir" '' else ""}
+        install -dm755 -o atd -g atd "$etcdir"
+        spool_and_job_dir_perms=${if cfg.allowEveryone then "1777" else "1770"}
+        install -dm"$spool_and_job_dir_perms" -o atd -g atd "$spooldir" "$jobdir"
         if [ ! -f "$etcdir"/at.deny ]; then
             touch "$etcdir"/at.deny
             chown root:atd "$etcdir"/at.deny