Fix overwriting of default CMake variants

[haikion]

Fixes the following issue:

Preconditions

• The project has at least one CMake option.
• The project contains a cmake-variants.yaml file with default variants such as debug. Example here: CMake Variants

Steps:

1. Edit an option to a non-default value within the settings section of cmake-variants.yaml.
2. Check the generated CMakeCache.txt.

Expected: The option change is reflected in CMakeCache.txt.
Actual: The change is not reflected.

Here is a small hello world project for easier testing: hello.zip

I noticed that cmake-tools generates a default argument list if the variant name happens to match one of the defaults. I would assume this is intended behavior when no YAML file exists, but it also occurs when the file is present. I tested vscode-cmake-tools, and there the settings change works correctly.

[lceWolf]

LGTM, thanks for your contribution