Add a few more bypass environment variables

When we're building ParticleOS images, we don't want the package
manager (or mkosi) to run systemd-sysusers, systemd-tmpfiles or
systemctl preset so let's add a few more bypass environment
variables that we can set to have execution of these skipped like
we already have $SYSTEMD_HWDB_UPDATE_BYPASS and $KERNEL_INSTALL_BYPASS.

src/sysusers/sysusers.c

@@ -38,6 +38,7 @@
 #include "uid-range.h"
 #include "user-util.h"
 #include "utf8.h"
+#include "verbs.h"
 
 typedef enum ItemType {
         ADD_USER =   'u',
@@ -2282,6 +2283,9 @@ static int run(int argc, char *argv[]) {
         if (arg_cat_flags != CAT_CONFIG_OFF)
                 return cat_config();
 
+        if (should_bypass("SYSTEMD_SYSUSERS"))
+                return 0;
+
         umask(0022);
 
         r = mac_init();