cmake.defaultVariants doesn't override default buildType, but appends

### Brief Issue Summary

When using the `cmake.defaultVariants` setting, the values used in the `buildType` entry are appended to the defaults instead of replacing them.

For example the following setting value:

```json
"cmake.defaultVariants": {
"buildType": {
"default": "beepbeep",
"description": "The build type.",
"choices": {
"beepbeep": {
"short": "Beep Beep",
"long": "This build goes beep beep",
"buildType": "Beep"
},
}
}
}
```

Gives the following list of variants:


This happens both with simple folder workspaces, and `.code-workspace` workspaces.

### CMake Tools Diagnostics

_No response_

### Debug Log

```shell
There is no additional debug info printed beyond "[variant] Loaded new set of variants"
```

### Additional Information

_No response_

Start by tracing how the cmake.defaultVariants setting is loaded and how the buildType entry is merged when the variant list is produced; the debug log identifies this as the "[variant] Loaded new set of variants" path. Verify the behavior for both folder and .code-workspace workspaces, and consider the issue done when custom buildType values replace rather than append to the defaults.

Written by the indexing model from the issue text.