sysusers: make sp_lstchg shadow field reproducible

If the environment variable SOURCE_DATE_EPOCH is set, use its value
instead of the current time.

src/basic/env-util.h

@@ -57,6 +57,8 @@ char *strv_env_pairs_get(char **l, const char *name) _pure_;
 int getenv_bool(const char *p);
 int getenv_bool_secure(const char *p);
 
+int getenv_uint64_secure(const char *p, uint64_t *ret);
+
 /* Like setenv, but calls unsetenv if value == NULL. */
 int set_unset_env(const char *name, const char *value, bool overwrite);