[gha] add PPA repository only on 20.04

.github/workflows/build.yml

@@ -13,9 +13,11 @@ jobs:
     - uses: actions/checkout@v2
       with:
         submodules: 'recursive'
+    - name: Build and push trunk container
+      if: matrix.os == 'ubuntu-20.04'
+      run: sudo add-apt-repository ppa:mhier/libboost-latest 
     - name: install packages
       run: |
-        sudo add-apt-repository ppa:mhier/libboost-latest
         sudo apt-get update
         sudo apt-get install build-essential qt5-default libqt5gui5 libboost1.74-dev libminiupnpc-dev libssl-dev zlib1g-dev
     - name: build application