How do you reference defined variables in a SQL Se

Question

I've read many questions on this such as:

https://social.msdn.microsoft.com/Forums/sqlserver/en-US/da4bdb11-fe42-49db-bb8d-288dd1bb72a2/sqlcmd-vars-in-create-table-script?forum=ssdt

and

How to run different pre and post SSDT pubish scripts depending on the deploy profile

What I'm trying to achieve is a way of defining a set of scripts based on the environment being deployed to. The idea is that the environment is passed in as a SQLCMD variable as part of the azure-devops pipeline into a variable called $(ServerName), which I've setup in the sql server database project under properties with a default of 'DEV'.

This is then used in the post deployment script like this:

:r .\PostDeploymentScripts\$(ServerName)\index.sql

This should therefore pick up the correct index.sql file based on the $(ServerName) variable. When testing this by publishing and entering 'QA' for the $(ServerName) variable and generating the script it was still displaying the 'DEV' scripts. However, the top of the script showed the variable had been set correctly:

How do I get the post deployment script to reference the $(ServerName) variable correctly so I can dynamically set the correct reference path?