Break out briefcase testing into its own job

.cirrus.yml

@@ -103,15 +103,18 @@ task:
   install_script:
     - >-
       apt-get update || true;
-      apt-get install -qq -y git python3 python3-pip
-    - python3 .ci/install-wheels.py
-    - pip install briefcase
+      apt-get install -qq -y git python3 python3-pip python3-venv
+    - python3 -m venv /py
+    - source /py/bin/activate; python3 .ci/install-wheels.py
+    - /py/bin/pip install briefcase
+
+  info_script:
+    - /py/bin/python --version
+    - /py/bin/pip list
 
   script:
-    - ${PY-python3} --version
-    - pip list
-    - briefcase new --no-input -Q bootstrap=PursuedPyBear
-    - cd helloworld && briefcase package
+    - /py/bin/briefcase new --no-input -Q bootstrap=PursuedPyBear
+    - cd helloworld && /py/bin/briefcase package
 
   use_compute_credits: $CIRRUS_BRANCH == 'staging'