Fix use-external-python in the action

fd01666
broke setting the Python version for the action.

While use-external-python works in
.github/workflows/test-python-install.yml, it does not work in the
action itself. This is because custom action inputs cannot use true
booleans, so the use-external-python input is a string type. This string
then always gets interpreted as "true", so the "Setup python" condition
always resolves to "false" due to negation.

Use fromJSON to correctly convert the strings "true" and "false" to
boolean types.

Author: RytoEX

action.yml

@@ -86,7 +86,7 @@ runs:
   using: "composite"
   steps:
     - name: Setup python
-      if: ${{ ! inputs.use-external-python }}
+      if: ${{ ! fromJSON(inputs.use-external-python) }}
       uses: actions/setup-python@v5
       with:
         python-version: ${{ inputs.python-version }}