[Security] Workflow master.yml is using vulnerable action actions/checkout

The workflow master.yml is referencing action actions/checkout using references v1. However this reference is missing the commit [a6747255bd19d7a757dbdda8c654a9f84db19839](https://github.com/actions/checkout/commits/a6747255bd19d7a757dbdda8c654a9f84db19839) which may contain fix to the some vulnerability.
The vulnerability fix that is missing by actions version could be related to:
(1) CVE fix
(2) upgrade of vulnerable dependency
(3) fix to secret leak and others.
Please consider to update the reference to the action.

Open master.yml and inspect its actions/checkout reference, which currently uses v1. Update the action reference to include commit a6747255bd19d7a757dbdda8c654a9f84db19839, then verify that the workflow remains valid and uses the requested revision.

Written by the indexing model from the issue text.