Activates venv before venv build

Also fixes toolchain.py failing silently on missing dependency.

.github/workflows/kivy_ios.yml

@@ -68,6 +68,7 @@ jobs:
         pip install Cython==0.28.1
     - name: Build Python & Kivy
       run: |
+        . venv/bin/activate
         python toolchain.py build python3 kivy
 
   build_updated_recipes:

toolchain.py

@@ -35,7 +35,7 @@ try:
 except ImportError:
     print("ERROR: Python requirements are missing")
     print("To install: pip install -r requirements.txt")
-    sys.exit(0)
+    sys.exit(1)
 curdir = dirname(__file__)
 sys.path.insert(0, join(curdir, "tools", "external"))