GString not flattened to String by DSL inside a map

I want to resolve the artifact to be unarchived at execution time.

Therefore I want to be able to perform the following:

requiredFile = 'target/<some archived file name>'

unarchive mapping: [requiredFile: .]

Unfortunately this does not replace `requiredFile` with its value.

trying to use the Groovy style like this:

unarchive mapping: ["$requiredFile": .]

results in

java.lang.ClassCastException: org.codehaus.groovy.runtime.GStringImpl cannot be cast to java.lang.String