Default -Dbuildtype=release cannot be overridden by native file

Nobody has claimed this yet.

I find it quite convenience to package together build options using Meson's [native files](https://mesonbuild.com/Native-environments.html). Unfortunately if I try to set the build type in a native file like e.g.:

```toml
[built-in options]
buildtype = 'debug'
```

this will not override the `-Dbuildtype` given by meson-python here (even though it's given earlier in the command line):

https://github.com/mesonbuild/meson-python/blob/cfcfb3554800d42c859d04309da2c1973c9f97ce/mesonpy/__init__.py#L874

I'm not sure whether this is something that should be fixed in Meson or meson-python. One workaround could be to put these defaults in a native file specified earlier in the command line. This should cause them to be overwritten by the user native file correctly.

Start in mesonpy/__init__.py around line 874 and reproduce the native-file buildtype behavior described in the issue. Compare how meson-python supplies -Dbuildtype with Meson's native-file precedence rules, then verify that a native file can provide the intended default without preventing explicit user settings from taking effect.

Written by the indexing model from the issue text.