fix azure linux image

Author: asizon

azure-pipelines.yml

@@ -17,6 +17,7 @@ variables:
   repoName: betaflight-configurator-nightlies
   debugReleaseNotes: This is a nightly build off the tip of 'master'. It may be unstable and result in corrupted configurations or data loss. **Use only for testing.**
   releaseNotes: This is a release build. It does not contain the debug console.
+  linuxVmImage: 'ubuntu-20.04'
 
 parameters:
   - name: releaseBuild
@@ -121,7 +122,7 @@ stages:
 
   - job: 'Linux'
     pool:
-      vmImage: 'ubuntu-20.04'
+      vmImage: '$(linuxVmImage)'
 
     steps:
     - task: UseNode@1