ensure poetry-exporter-plugin is installed

Author: Cube707

.github/workflows/tests.yml

@@ -55,7 +55,7 @@ jobs:
 
       - name: Install Poetry
         run: |
-          pipx install --pip-args=--constraint=.github/workflows/python-constraints.txt poetry
+          pipx install --pip-args=--constraint=.github/workflows/python-constraints.txt poetry poetry-plugin-export
           poetry --version
 
       - name: Install Nox