Use hashed requirements lock

.github/workflows/briefcase.yml

@@ -16,8 +16,10 @@ jobs:
       - name: Install dependencies
         run: |
           python -m pip install --upgrade pip
-          pip install -r src/requirements.txt
-          pip install briefcase
+          pip install pip-tools briefcase
+          pip-compile --generate-hashes --output-file=requirements.lock src/requirements.txt
+          git diff --exit-code requirements.lock
+          pip install --require-hashes -r requirements.lock
       - name: Build with Briefcase
         run: briefcase build
       - name: Upload artifacts