Setup poetry before snyk test

.github/workflows/security-scan.yml

@@ -21,6 +21,10 @@ jobs:
         steps:
             - name: Checkout Repository
               uses: actions/checkout@v4
+            - uses: snok/install-poetry@v1
+              with:
+                  virtualenvs-create: true
+                  virtualenvs-in-project: true
             - run: poetry install
               working-directory: backend
             - uses: snyk/actions/setup@master