community/mustach: disable valgrind in tests on s390x

Running the tests with valgrind resulted in a segfault on the 3.21 s390x
builder. Disable use of valgrind in tests on the arch until resolved.

```
==8545== Memcheck, a memory error detector
==8545== Copyright (C) 2002-2024, and GNU GPL'd, by Julian Seward et al.
==8545== Using Valgrind-3.23.0 and LibVEX; rerun with -h for copyright info
==8545== Command: ../mustach json must
==8545==

valgrind: m_debuginfo/image.c:700 (get_slowcase): Assertion 'i <= img->ces_used' failed.
Segmentation fault
```