Support both master and main in Python (Azure#18074)

Author: sima-zhu

eng/pipelines/aggregate-reports.yml

@@ -4,6 +4,7 @@ pr:
   branches:
     include:
       - master
+      - main
   paths:
     include:
       - eng/pipelines/aggregate-reports.yml

eng/pipelines/autorest_checks.yml

@@ -5,6 +5,7 @@ pr:
   branches:
     include:
     - master
+    - main
     - feature/*
     - hotfix/*
     - release/*

eng/pipelines/templates/jobs/tests-nightly-python.yml

@@ -1,5 +1,6 @@
 trigger:
   - master
+  - main
 
 jobs:
 

eng/pipelines/templates/jobs/update_pr.yml

@@ -5,6 +5,7 @@
 
 trigger:
 - master
+- main
 
 variables:
   skipComponentGovernanceDetection: true

eng/pipelines/templates/steps/build-artifacts.yml

@@ -24,11 +24,11 @@ steps:
       PackageName: "azure-template"
       ServiceDirectory: "template"
       TestPipeline: ${{ parameters.TestPipeline }}
-
+  - template: /eng/common/pipelines/templates/steps/set-default-branch.yml
   - script: |
       echo "##vso[build.addbuildtag]Scheduled"
     displayName: 'Tag scheduled builds'
-    condition: and(eq(variables['Build.SourceBranchName'],'master'),eq(variables['Build.Reason'],'Schedule'))
+    condition: and(eq(variables['Build.SourceBranchName'], variables['DefaultBranch']), eq(variables['Build.Reason'],'Schedule'))
 
   - task: UsePythonVersion@0
     displayName: 'Use Python 2.7'