fix: doesn't work with virtual environment's `sqlfluff` even if on $PATH

[dijonkitchen]

For some reason, even though terminal can find sqlfluff on the $PATH, the extension can't find it by default when VS Code auto-detect pipenv virtual environment (potentially all virtual environments).

The sqlfluff executable was not found. Use the 'Executable Path' setting to configure the location of the executable, or add it to your PATH.

I can get it to work with an absolute path, but this is not sharable with others on team using their own specific virtual environments that may have custom hash in name.