github: use pip from GHA runner image

Signed-off-by: Simon Deziel <simon.deziel@canonical.com>

.github/workflows/tests.yml

@@ -56,9 +56,7 @@ jobs:
         sudo adduser "$USER" lxd
         sudo lxd init --auto
 
-        sudo apt-get update
-        sudo apt-get install python3-pip
-        pip install --upgrade pip tox codecov
+        pip install --upgrade tox codecov
 
     - name: Coverage
       run: |