This is a more generic version of this question: How to run a build step on a specific branch only?
For example, I can use a PowerShell script to run MSBuild if '%teamcity.build.branch.is_default%' -eq 'true' or if '%teamcity.build.branch%' -eq 'master' but then I will miss the collapsible log that comes with the TeamCity MSBuild build runner. Isn't there any easier way to conditionally run a build step?
It is not possible to execute build step based on condition. Vote for the related request: https://youtrack.jetbrains.com/issue/TW-17939.
The recommended approach is to create separate build configuration for each branch. You can use templates to simplify the setup. In this case it will be easier to interpret the results and the statistics of the builds will be informative.
Also see the related answer (hack is suggested).