[CI] Switch GitHub Actions to Ubuntu 20.04 (#4582)

Author: Alexander Batashev

.github/workflows/linux_post_commit.yml

@@ -6,7 +6,7 @@ on:
       - sycl
 jobs:
    check:
-     runs-on: ubuntu-18.04
+     runs-on: ubuntu-20.04
      if: github.repository == 'intel/llvm'
      strategy:
        fail-fast: false
@@ -28,7 +28,7 @@ jobs:
                  ;;
              SharedLibs)
                  wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-                 sudo add-apt-repository "deb http://apt.llvm.org/bionic/   llvm-toolchain-bionic-13 main"
+                 sudo add-apt-repository "deb http://apt.llvm.org/focal/   llvm-toolchain-focal-13 main"
                  sudo apt-get update
                  sudo apt-get install -y clang-13
                  export ARGS="--shared-libs"