Fix jarFileTest cache relocatability

The jarFileTest task sets `systemProperty("jarFile", absolutePath)` which
embeds an absolute path as a task input, making the task not
cache-relocatable across different project locations.

Fix by:
- Using `inputs.files(shadowJar).withPathSensitivity(RELATIVE)` instead
  of the bare `file()` call (which doesn't register a proper input)
- Passing the jar path as a relative path to the system property

The test (AbstractJarFileTest) uses `Paths.get(System.getProperty("jarFile"))`
which resolves relative paths against the working directory (project dir),
so the test works identically with both absolute and relative paths.

Author: ribafish

core/build.gradle

@@ -34,8 +34,8 @@ task jarFileTest(type: Test) {
     testClassesDirs = sourceSets.jarFileTest.output.classesDirs
     classpath = sourceSets.jarFileTest.runtimeClasspath
 
-    file(shadowJar.outputs.files.singleFile) // input for correct caching
-    systemProperty("jarFile", shadowJar.outputs.files.singleFile)
+    inputs.files(shadowJar).withPathSensitivity(PathSensitivity.RELATIVE)
+    systemProperty("jarFile", project.projectDir.toPath().relativize(shadowJar.outputs.files.singleFile.toPath()).toString())
 
     dependsOn(shadowJar)
 }