GitLab

meson-python does not like duplicate ARCHFLAGS values, and will error
with output such as:

```
('\x1b[31m',)meson-python: error: Multi-architecture builds are not
supported but $ARCHFLAGS='-arch x86_64 -arch x86_64'
[end of output]
```

https://github.com/nipy/nipy-wheels/actions/runs/6805528295/job/18505232990#step:4:164
at time of writing.

When appending to ARCHFLAGS. Otherwise it's easy to see clang fail because of `-arch arm64-arch arm64`.