ci: Run test on ubuntu-20.04 due to lack of snap7 on newer ubuntu

.github/workflows/test.yml

@@ -14,14 +14,14 @@ jobs:
       DOTNET_NOLOGO : 1
     strategy:
       matrix:
-        os: [windows-latest, ubuntu-latest, macos-latest]
+        os: [windows-latest, ubuntu-20.04, macos-latest]
         test-framework: [netcoreapp3.1, net5.0]
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             test-framework: netcoreapp3.1
             installSnap7: true
             dotnet-sdk: '3.1.x'
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             test-framework: net5.0
             installSnap7: true
             dotnet-sdk: '5.0.x'
@@ -48,7 +48,7 @@ jobs:
       - uses: actions/checkout@v2
 
       - name: Install Snap7 Linux
-        if: ${{ matrix.installSnap7 && matrix.os == 'ubuntu-latest' }}
+        if: ${{ matrix.installSnap7 && matrix.os == 'ubuntu-20.04' }}
         run: |
           sudo add-apt-repository ppa:gijzelaar/snap7
           sudo apt-get update