chg: ci: Use meson native files for CI build configure flags

Make it easier to reproduce failed CI builds locally by using meson native files to configure the jobs, rather than composing variables in `.gitlab-ci.yml`.

To reproduce a build locally, the following can be used now: (with `$EXTRA_SETUP_FILES` being set per-job in `.gitlab-ci.yml`, single variable, no composition)

```
meson setup --native-file ci/common.ini $EXTRA_SETUP_FILES build
```

Closes #5470

Merge branch '5470-meson-native-files' into 'main'

See merge request isc-projects/bind9!12357