I'd create an ivysettings.xml file and specify the location of my cache using the caches directive:
<ivysettings>
<settings defaultResolver="central"/>
<caches defaultCacheDir="${ivy.settings.dir}/cache"/>
<resolvers>
<ibiblio name="central" m2compatible="true"/>
</resolvers>
</ivysettings>
I think this more explicit and is less cryptic than setting the property ivy.default.ivy.user.dir within your build file.
Update
Using this approach the ivy cleancache task can be used to purge your nominated cache directory.
<target name="clean-all" depends="clean">
<ivy:cleancache />
</target>
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