Make 'source-root' init input relative to github.workspace

In the previous commit, the default value of the input is ${{ github.workspace }}
which means that most uses of this input would probably prefix their paths with
${{ github.workspace }}, especially since actions/checkout's 'path' input
must be under ${{ github.workspace }}. Therefore, it doesn't make much sense for
this to be an absolute file path.

Instead, it's more intuitive to make this relative to the repository.

init/action.yml

@@ -39,9 +39,8 @@ inputs:
     required: true
     default: 'true'
   source-root:
-    description: The root source-code directory.
-    required: true
-    default: ${{ github.workspace }}
+    description: Path to the root source-code directory, relative to ${{ github.workspace }}.
+    required: false
 outputs:
   codeql-path:
     description: The path of the CodeQL binary used for analysis