What is the windows equivalent for envsubst in linux

I don't directly know of a built in substitute for the envsubst command in Windows. However as you've made 3rd party tools an option, I would recommend MinGW, a minimalist GNU package for Windows users. It also comes with a envsubst implementation for Windows.

CygWin could also be an option. It provides a more complete collection of GNU tools and other Open Source tools for Windows (comes also with a POSIX API for Windows for example). It should also have an envsubst but as I don't have CygWin locally I cannot confirm this.

I would go for MinGW though as the extra functionnalities CygWin provides (POSIX environment) are not needed for envsubst but the choice is up to you (this SO post and this article may help you make a decision).