Name already in use

This input is exposed in the CodeQL CLI as the flag --source-root, allowing
users of the CLI to set --source-root different from --working-dir. However,
in codeql-action, these two paths are conflated and it poses problems for
users with complicated build environments, in which a source root may be
a child of the working directory.

Most users should not notice this, as the default value is
${{ github.workspace }}, as it is implied now (`path.resolve()`).