Previously, we were adding dependencies to Ant's ClassLoader within
Gradle. It is suspected that this was causing sporadic loader
contraint violations as types that Gradle itself uses (from Commons
Compress) were then available from two different ClassLoaders.

This commit reworks the Ant smoke test to use JavaExec and Ant's
launcher to run the build. This allows us to make the necessary
dependencies available to Ant in an isolated manner. The javac
invocation within Ant is now forked to allow it to find the tools jar
even when the build itself is running on a JRE.

Closes gh-19839