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.

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