.github/workflows/main.yml: run static-analysis on bionic

GitHub Actions is transitioning workflow steps that run on
'ubuntu-latest' from 18.04 to 20.04 [1].

This works fine in all steps except the static-analysis one, 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.

[1]: actions/runner-images#1816

Reported-by: Junio C Hamano <[email protected]>
Signed-off-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: ttaylorr

azure-pipelines.yml

@@ -473,7 +473,7 @@ jobs:
   displayName: StaticAnalysis
   condition: succeeded()
   pool:
-    vmImage: ubuntu-latest
+    vmImage: ubuntu-18.04
   steps:
   - bash: |
        test "$GITFILESHAREPWD" = '$(gitfileshare.pwd)' || ci/mount-fileshare.sh //gitfileshare.file.core.windows.net/test-cache gitfileshare "$GITFILESHAREPWD" "$HOME/test-cache" || exit 1