Improve the PR build version (sonic-net#7381)

Why I did it
Improve the version of the Pull Request build by changing the local branch name.

How I did it
Change the default branch name merge to [target_branch_name]-[pullrequestid].

How to verify it
For official build, the version is not changed.
For pull request build, the version as below:

.azure-pipelines/azure-pipelines-image-template.yml

@@ -38,7 +38,9 @@ jobs:
         submodules: recursive
         displayName: 'Checkout code'
       - script: |
-          git checkout -b $(Build.SourceBranchName)
+          BRANCH_NAME=$(Build.SourceBranchName)
+          [ -n "$SYSTEM_PULLREQUEST_PULLREQUESTID" ] && BRANCH_NAME="$SYSTEM_PULLREQUEST_TARGETBRANCH-$SYSTEM_PULLREQUEST_PULLREQUESTNUMBER"
+          git checkout -b $BRANCH_NAME
           sudo modprobe overlay
           sudo apt-get install -y acl
           sudo bash -c "echo 1 > /proc/sys/vm/compact_memory"