Fix pip invocation in .cirrus.yml

.cirrus.yml

@@ -13,7 +13,7 @@ iocage_tests_task:
   pip_cache:
     folder: ~/.cache/pip
     populate_script:
-      - python3 install -r requirements-test.txt
+      - python3 -m pip install -r requirements-test.txt
       - python3 -m pip install -U cython
   env_setup_script:
     - python3 -m pip install -U -r requirements-test.txt