util: make GMT timestamps explicit for clarity

For privacy reasons, time functions use GMT, to avoid logs leaking
timezones. It'd make more sense to use localtime for wallet output
(which are not logged by default), but that adds inconsistencies
which can also be confusing. So add a Z suffix for now to make it
clear these are not local time.

src/common/util.cpp

@@ -1067,7 +1067,7 @@ std::string get_nix_version_display_string()
     time_t tt = ts;
     struct tm tm;
     misc_utils::get_gmt_time(tt, tm);
-    strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%S", &tm);
+    strftime(buffer, sizeof(buffer), "%Y-%m-%d %H:%M:%SZ", &tm);
     return std::string(buffer);
   }