Removed usage of ubuntu-18.04 from github actions.

Author: arobenko

.github/workflows/actions_build.yml

@@ -172,8 +172,8 @@ jobs:
         VERBOSE: 1 
 
 
-  build_clang_ubuntu_18_04:
-    runs-on: ubuntu-18.04
+  build_clang_old_ubuntu_20_04:
+    runs-on: ubuntu-20.04
     strategy:
       fail-fast: false
       matrix:
@@ -187,6 +187,11 @@ jobs:
     steps:
     - uses: actions/checkout@v2
 
+    - name: Add repositories
+      run: |
+        sudo add-apt-repository 'deb http://archive.ubuntu.com/ubuntu/ bionic main'; \
+        sudo add-apt-repository 'deb http://archive.ubuntu.com/ubuntu/ bionic universe'         
+
     - name: Prepare Install
       run: sudo apt-get update --fix-missing