Refer to $GITHUB_WORKSPACE specifically in source-root description

Previously, I had tried to refer to '${{ github.workspace }}', but that caused a problem in Actions. Trying to avoid the issue, I changed this to "the workspace," but this gives up quite a bit of specificity.

Co-authored-by: Arthur Baars <aibaars@github.com>

init/action.yml

@@ -39,7 +39,7 @@ inputs:
     required: true
     default: 'true'
   source-root:
-    description: Path to the root source-code directory, relative to the workspace.
+    description: Path of the root source code directory, relative to $GITHUB_WORKSPACE.
     required: false
 outputs:
   codeql-path: