Update brew before installing packages

brew changed his backend repository, we must update brew itself first.

.github/workflows/ci.yml

@@ -13,7 +13,8 @@ jobs:
         with:
           python-version: '3.5'
       - name: Install packages
-        run:
+        run: |
+          brew update
           brew install gcovr pkg-config ninja
       - name: Install python modules
         run: pip3 install meson==0.49.2 pytest