Explicitly install clang-9 in CI ubuntu images

GitHub removed default installation of clang-9 from the ubuntu
images but those are required to build postgres with llvm so we have
to explicitly install the required packages.

actions/runner-images#3381

.github/workflows/linux-build-and-test.yaml

@@ -64,7 +64,7 @@ jobs:
         key: ${{ matrix.os }}-postgresql-${{ matrix.pg }}-${{ matrix.cc }}-${{ matrix.build_type }}${{ env.CACHE_SUFFIX }}
 
     - name: Build PostgreSQL ${{ matrix.pg }} ${{ matrix.build_type }}
-      if: steps.cache-postgresql.outputs.cache-hit != 'true'
+#      if: steps.cache-postgresql.outputs.cache-hit != 'true'
       run: |
         wget -q -O postgresql.tar.bz2 https://ftp.postgresql.org/pub/source/v${{ matrix.pg }}/postgresql-${{ matrix.pg }}.tar.bz2
         mkdir -p ~/$PG_SRC_DIR

scripts/gh_matrix_builder.py

@@ -49,6 +49,7 @@ def build_debug_config(overrides):
     "tsdb_build_args": "-DCODECOVERAGE=ON -DWARNINGS_AS_ERRORS=ON",
     "installcheck_args": "IGNORES='bgw_db_scheduler'",
     "coverage": True,
+    "extra_packages": "clang-9 llvm-9 llvm-9-dev llvm-9-tools",
     "llvm_config": "llvm-config-9",
     "clang": "clang-9",
     "os": "ubuntu-20.04",
@@ -100,6 +101,7 @@ def macos_config(overrides):
     "llvm_config": "/usr/local/opt/llvm/bin/llvm-config",
     "coverage": False,
     "installcheck_args": "IGNORES='bgw_db_scheduler bgw_launcher remote_connection'",
+    "extra_packages": "",
   })
   base_config.update(overrides)
   return base_config