Azure: Install compiler explicitely

Author: Flamefire

.azure-pipelines.yml

@@ -153,7 +153,9 @@ stages:
               displayName: Get CCache
             - bash: |
                 set -ex
-
+                if ! command -v "$B2_COMPILER" &> /dev/null && [[ "${{item.os}}" == "ubuntu"* ]]; then
+                  sudo apt-get -o Acquire::Retries=$NET_RETRY_COUNT -y -q --no-install-suggests --no-install-recommends install "$B2_COMPILER"
+                fi
                 for ((i=1; i <= NET_RETRY_COUNT; i++)); do
                   git clone --depth 1 --branch master https://github.com/boostorg/boost-ci.git boost-ci-cloned && break || sleep 10
                 done