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/tmpfiles/tmpfiles.c

@@ -71,6 +71,7 @@
 #include "terminal-util.h"
 #include "umask-util.h"
 #include "user-util.h"
+#include "verbs.h"
 #include "virt.h"
 #include "xattr-util.h"
 
@@ -4618,6 +4619,9 @@ static int run(int argc, char *argv[]) {
         if (arg_cat_flags != CAT_CONFIG_OFF)
                 return cat_config(config_dirs, argv + optind);
 
+        if (should_bypass("SYSTEMD_TMPFILES"))
+                return 0;
+
         umask(0022);
 
         r = mac_init();