Fix jarFileTest cache relocatability (#11574)

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.