Handle GString compiler args in Gradle plugin

[jupblb]

Fixes #801

Groovy build scripts that append interpolated strings to compiler args (options.compilerArgs << "${...}") put GStringImpl objects into the list. The plugin's duplicate-check args.stream().anyMatch(arg -> arg.startsWith("-Xplugin:scip")) on a List<String> inserts an erasure-driven checked cast to String per element, throwing:

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

The fix iterates compilerArgs through a List<?> reference and compares via String.valueOf(arg), so no element is ever cast to String. (Casting inside the lambda body would not help: the erased Predicate.test(Object) bridge casts the argument before the body runs.)

Includes a regression fixture (gradle/gstring-compiler-args) that appends a GString to compilerArgs; without the fix it fails with the exact error from the issue, with the fix all GradleBuildToolTest tests pass.