Resolve registerDependencies output lazily so a custom build directory applies

[jjh75607]

Fixes #2114

setup() resolved the build directory eagerly and stored a plain File behind @OutputFile.
A layout.buildDirectory set later in the build script was therefore ignored. The marker file
stayed under the default build/.

getUnitOutput() is now a RegularFileProperty fed from getBuildDirectory().file(...), so
it resolves at execution time. The field was only read inside this task, so nothing else
changes.

One note on the reproduction. On current main this task is only registered by
SpotlessExtensionPredeclare. The test therefore drives it through spotlessPredeclare, not
the plain spotless { java {} } from the original report. I could not reproduce it without
predeclare.