Replace `com.Expand`

[jolheiser]

Here is a search (on GH) of our usage of com.Expand.

We should think about replacing it with either template execution or os.Expand.

There isn't anything necessarily wrong with com.Expand, but an stdlib implementation would be nice, and it would be manageable to refactor now.

[wxiaoguang]

Finally, it's done in

• Refactor legacy unknwon/com package, improve golangci lint #19284

[lunny]

A little problem is the syntax between vars/expand and os/expand is not the same. So that readme init placeholder syntax is not the same as template repository.