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
```

community/mustach/APKBUILD

@@ -20,9 +20,10 @@ for _j in $_jsonlibs; do
 done
 
 # armv7: Segmentation fault (valgrind error #15759)
+# s390x: Segmentation fault (valgrind error on 3.21 builder)
 # armhf,riscv64,loongarch64: doesn't have valgrind
 case "$CARCH" in
-armv7|armhf|riscv64|loongarch64) export NOVALGRIND=1;;
+armv7|s390x|armhf|riscv64|loongarch64) export NOVALGRIND=1;;
 *) checkdepends="valgrind";;
 esac