Bamboo Variables are not expanded in MSTest results path definition

In a MSBuild/NANT/devenv.com builder, if the Test Result Files location include a Bamboo variable, the variable is not expanded and bamboo will not be able to find the file.

Example:

[AbstractDotNetBuilder] Processing test results in:E:\bamboo-home\xml-data\build-dir\131073\PROJ-PLAN-JOB1 with path: TestResults-${bamboo.buildNumber}.trx