Customizing CMAKE_CXX_FLAGS is not possible with codeql-cli auto-build

I have encountered an issue when using the default auto-build feature, as it does not pass the CMAKE_CXX_FLAGS parameter to cmake. Upon closer inspection, I found that the `cpp/tools/do-prebuild.sh` script uses a fixed `CXXFLAGS=-fpermissive`.

It would be beneficial if the script could allow reading the CXXFLAGS configuration from environment variables, like this:

```bash
export CXXFLAGS="'${CXXFLAGS:-} -fpermissive'"
```
```[tasklist]
### Tasks
```

Start with cpp/tools/do-prebuild.sh and inspect how the auto-build invokes CMake and sets CXXFLAGS. Run the CodeQL CLI auto-build with a custom CMAKE_CXX_FLAGS value; it is done when the configured flags reach CMake while -fpermissive is still included.

Rédigé par le modèle d'indexation à partir du texte de l'issue.