ci: call pip correctly

.gitlab-ci.yml

@@ -21,8 +21,8 @@ build-openapi-scheme:
     # we do this, because YAML anchors are not supported when executing locally
     - apt-get update
     - apt-get install -y gcc libldap2-dev libsasl2-dev libmagic-dev
-    - pip install --upgrade pip
-    - pip install poetry==1.8.1
+    - python3 -m pip install --upgrade pip
+    - python3 -m pip install poetry==1.8.1
     - poetry install --without dev,test --no-root
   script:
     - mkdir public