fix venv activate

Author: FoamyGuy

Diff for: .github/workflows/build-images.yml

@@ -14,7 +14,10 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - run: python3 -m venv build_images_venv
-    - run: source build_images_venv/bin/activate
+    - name: Activate virtualenv
+      run: |
+        . .build_images_venv/bin/activate
+        echo PATH=$PATH >> $GITHUB_ENV
     - run: python3 -mpip install -r requirements.txt
     - run: git clone --depth=1 https://github.com/adafruit/Adafruit_Learning_System_Guides learn
     - run: env LEARN_GUIDE_REPO=learn/ python3 create_requirement_images.py