Windows tool paths require `.exe` when passed to CMake

CMake does not accept paths to tools which look like `c:/msys64/mingw64/bin/gcc`, reporting "... is not a full path to an existing compiler tool."

It needs to have ".exe" appended to the tool names. Not sure how this hasn't been spotted yet but here we are :smile:. This is probably good practise for passing tools to any underlying build system on Windows.

No source file or test is named. Start by locating the CMake tool-path handling and reproduce the reported Windows case with a compiler path lacking `.exe`; done means CMake accepts the resulting tool paths without the existing-compiler error.

Written by the indexing model from the issue text.