Explicitly install g++-9 on linux images (microsoft#7889)

This is no longer included by default on the build images.

Author: damyanp

azure-pipelines.yml

@@ -133,6 +133,11 @@ stages:
       displayName: 'Installing dependencies'
       condition: eq(variables['image'], variables['linux'])
 
+    - bash: |
+        sudo apt-get install g++-9 -y
+      displayName: 'Installing g++-9'
+      condition: and(eq(variables['CXX'], 'g++-9'), eq(variables['image'], variables['linux']))
+
     - bash: |
         brew update
         brew install ninja