azure-pipeline: run static-analysis on bionic

This is a backport of d051ed7 (.github/workflows/main.yml: run
static-analysis on bionic, 2021-02-08) to the Azure Pipeline.

When Azure Pipelines' build agents transitioned 'ubuntu-latest' from
18.04 to 20.04, it broke our `static-analysis` job, since Coccinelle
isn't available on Ubuntu focal (it is only available in the universe
suite).

Until Coccinelle can be installed from 20.04's main suite, pin the
static-analysis build to run on 18.04, where it can be installed by
default.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

azure-pipelines.yml

@@ -365,7 +365,7 @@ jobs:
   displayName: StaticAnalysis
   condition: succeeded()
   pool:
-    vmImage: ubuntu-latest
+    vmImage: ubuntu-18.04
   steps:
   - bash: |
        sudo apt-get update &&