GitHub Actions: Update apt index before installing packages

.github/workflows/build.yml

@@ -37,6 +37,8 @@ jobs:
       - uses: actions/checkout@v2
       - name: Install prerequisites
         run: >-
+          sudo apt-get update
+
           sudo apt-get install -y
           build-essential
           cmake
@@ -95,6 +97,8 @@ jobs:
           apt-get update
       - name: Install prerequisites
         run: >-
+          apt-get update
+
           apt-get install -y
           git
           cmake
@@ -145,6 +149,8 @@ jobs:
       - uses: actions/checkout@v2
       - name: Install prerequisites
         run: >-
+          brew update
+
           brew install
           cmake
           flex