Add 'source-root' input to init action

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()`).

init/action.yml

@@ -38,6 +38,10 @@ inputs:
     description: Try to auto-install your python dependencies
     required: true
     default: 'true'
+  source-root:
+    description: The root source-code directory.
+    required: true
+    default: ${{ github.workspace }}
 outputs:
   codeql-path:
     description: The path of the CodeQL binary used for analysis