CI: skip valgrind test on Arch

It's already run on other distros. This one fails though, due to missing
debug info.

```
valgrind:  Possible fixes: (1, short term): install glibc's debuginfo
valgrind:  package on this machine.  (2, longer term): ask the packagers
valgrind:  for your Linux distribution to please in future ship a non-
valgrind:  stripped ld.so (or whatever the dynamic linker .so is called)
```

It doesn't seem possible to have this work out of the box. The debuginfo
packages aren't reliably available, and debuginfod servers -- even if
they worked, which they apparently don't -- would not help anyway since
old version pruning can result in symbols disappearing before the image
is rebuilt, and thereby causing failure.

It's not really critical to test this, since as mentioned we already
have coverage of Meson's side in other distro ciimages.

ci/ciimage/arch/install.sh

@@ -13,7 +13,7 @@ pkgs=(
   mono boost qt5-base gtkmm3 gtest gmock protobuf gobject-introspection
   itstool gtk3 java-environment=8 gtk-doc llvm clang sdl2 graphviz
   doxygen vulkan-validation-layers openssh mercurial gtk-sharp-2 qt5-tools
-  libwmf valgrind cmake netcdf-fortran openmpi nasm gnustep-base gettext
+  libwmf cmake netcdf-fortran openmpi nasm gnustep-base gettext
   python-lxml hotdoc rust-bindgen qt6-base qt6-tools wayland wayland-protocols
   # cuda
 )