Fix signature key errors in snap action

.github/workflows/snap.yml

@@ -59,6 +59,9 @@ jobs:
 
       - name: First set up.
         run: |
+          # Workaround for permanent problems with third-party repository keys
+          sudo rm -rf /etc/apt/sources.list.d/*
+
           sudo apt-get update
           sudo apt-get install gcc-8 g++-8 -y
           sudo snap install --classic snapcraft