Docs build in post-submit fails fairly frequently due to racing updates

### description

if there are 2 or more updates to main in relatively quick succession the docs build can fail as during the period between checking out the existing docs from https://github.com/envoyproxy/envoyproxy.github.io and finishing the docs build the HEAD commit hash can have changed and the second or subsequent commits to that repo are rejected

probably the solution is to merge or rebase the website checkout after the docs build on main

it happens fairly frequently and can result in missing docs when it happens on a release branch