Sandbox: Permit the clone3 system call

Apparently glibc-2.34 uses clone3, when previously it just used
clone.

Closes ticket #40590.

changes/clone3-sandbox

+  o Minor features (linux seccomp2 sandbox):
+    - Permit the clone3 syscall, which is apparently used in glibc-2.34 and
+      later. Closes ticket 40590.

src/lib/sandbox/sandbox.c

@@ -144,6 +144,9 @@ static int filter_nopar_gen[] = {
     SCMP_SYS(clock_gettime),
     SCMP_SYS(close),
     SCMP_SYS(clone),
+#ifdef __NR_clone3
+    SCMP_SYS(clone3),
+#endif
     SCMP_SYS(epoll_create),
     SCMP_SYS(epoll_wait),
 #ifdef __NR_epoll_pwait